A typical lithium-ion battery (LIB) consists of a lithium-free negative electrode (graphite, Si/C, etc.), a lithium-containing positive electrode (LiFePO4 (LFP), LiCoO2 (LCO), and LiNixCoyMnzO2 (NCM), etc.) The Li intercalation mechanism paves the way for LIBs with excellent properties.
When actually purchasing and applying lithium batteries such as lithium golf cart batteries and fish finder batteries, it is necessary to evaluate electrochemical indicators such as specific capacity, voltage window, and current density. The ideal combination of these metrics leads to the best performance of LIBs, which is crucial for their commercialization in different fields.
Notably, coulombic efficiency (CE), defined as the ratio of discharge capacity to charge capacity, is a key metric, as it quantifies the battery loss capacity per cycle, which further predicts LIB lifetime. According to “capacity retention rate = (CE)n”, the average CE of the battery needs to be at least 99.99% to achieve 90% capacity retention rate for 1000 cycles. In a closed system, active lithium ions originate from the limited lithium source of the lithium-containing cathode and cannot be replenished, so the lower average CE means that the capacity of LIBs continues to decay throughout cycling.
Furthermore, special attention should be paid to the initial CE (ICE) considering the formation of a solid-electrolyte interface (SEI) film during initial charging, which consumes 5–10% of Li ions from Li-containing cathodes. Li+ battery loss are even higher for Si-based anodes (15-35%) and other anodes with large volume changes and large specific surface areas. Although the battery loss of lithium can be offset by excess cathode loading, the inactive weight in the battery reduces its overall energy density.
The need for low cost and high energy density has accelerated the development of high-capacity cathode materials, such as ultra-high Ni-rich or Li-rich layered oxide cathodes with high energy densities (>800 Whkg-1).
Reducing the Co content can reduce the cost of the cathode; However, Li+/Ni2+ antisite defects, high catalytic surface, and poor thermal stability limit the electrochemical performance of Ni-rich, Li-rich cathodes for delithiation. Advances in electrode materials have brought the energy density of Li-ion batteries close to the limit, which has prompted further optimization of Li-ion batteries to reach the near-term key energy density target of 350Wh kg-1.
To maximize energy density, Li metal batteries have received great attention in the past five years and have achieved unprecedented progress, which is manifested in high Li metal deposition/stripping efficiencies of >99.2% and ∼350Wh/kg High energy density (battery grade). However, the safety issues caused by dendrite formation cannot be fundamentally resolved, limiting its application to a niche market.
On the other hand, advanced battery chemistries such as lithium-sulfur batteries (LSBs) and Li-O2 batteries (LOBs) with higher specific capacity and energy density have also attracted great research interest. Despite the different Li+ storage mechanisms, Li metal-free LSBs and LOBs suffer from the same problems, namely low ICE, battery loss capacity, etc. To meet the key requirements for commercial applications, some strategies for mitigating battery loss capacity (MCL) in LSBs and LOBs have also been reported, proposing cost-effective prospects for obtaining high-energy batteries.
Development history of secondary battery performance. The green liquid in the battery represents the aqueous electrolyte and the clear liquid represents the non-aqueous electrolyte
Reasons for lithium battery loss
Initial capacity battery loss
Currently, none of the electrolytes are thermodynamically stable within the operating potential range of LIBs. The formation of SEI during the initial cycle constitutes the basis for the subsequent normal operation of lithium batteries, which leads to the consumption of a large amount of Li+ ions therein, which accounts for a large portion of the initial battery loss capacity.
In the 1960s, studies on interphase phenomena between alkali metals and non-aqueous systems began. In the 1970s, Peled et al. proposed a classic and general model to explain surface films formed on all alkali/alkaline earth metals in non-aqueous solutions. The decomposition of solvated ions and solvents yields inorganic compounds such as LiF, Li2O, and Li2CO3 as well as partially soluble hemicarbonates and polymers, which constitute SEIs with high resistivity.
Since the SEI is generated in situ, the initial battery loss capacity is related to the electrode state and the properties of the SEI (such as specific surface area, film thickness, etc.). A silicon anode was used to estimate the battery loss capacity due to SEI formation.
They concluded that the battery loss capacity was in the range of 0.06-0.1 C cm-2 (0.0178 and 0.0278 mA h cm-2). It is demonstrated that the solvent reduction at the carbon/SEI interface increases the thickness of the SEI, resulting in a battery loss capacity of about 10%. Therefore, high surface area and thick SEI films usually lead to significant battery loss of active Li+.
LIB battery loss capacity (a) Schematic diagram of the SEI film formed on the surface of the negative electrode and its composition. (b) Reasons for continued battery loss capacity in LIBs.
Sustained battery loss capacity
Sustained battery loss capacity occurs during repeated cycling, primarily due to: Continuous SEI formation or dendrite growth. As shown in Figure 2a, SEI can prevent the co-intercalation of Li+ ions and solvent into the graphite layer, maintaining the structural integrity of the graphite anode.
Even so, the rupture of the SEI film still occurs locally on the negative electrode surface, resulting in cracks. The growth and healing of the SEI film further consumes the limited lithium source. This situation is more serious for anodes with large volume changes during lithiation and delithiation.
The continuous SEI formation thickens the SEI and increases the internal resistance of the cell. The lithium deposition process at the negative electrode is an undesirable process that occurs if the rate of charge exceeds the rate at which lithium ions can be inserted into the negative electrode.
The poor lithium deposition/stripping efficiency in conventional carbonate electrolytes exacerbates the irreversible lithium ion battery loss. In addition, lithium dendritic deposits can cause internal short circuits, leading to safety concerns. It should be noted that the high internal resistance of the battery is more prone to lithium deposition. Electrode powdering.
Related to active materials, especially Si, Ge, Sn, etc., can lead to electrode dusting. Once these broken fragments are fully encapsulated by SEI, they cannot participate in the subsequent lithiation/delithiation reactions.
This greatly accelerates the capacity fading, resulting in severe Li+ loss. In addition, the pulverization of the alloy anode leads to the exfoliation of the active material from the current collector, which is common for bulk Si thin films and micron-sized Si particles. The nanostructured Si anode exhibits improved cycling stability due to rapid stress relaxation and structural fusion. phase transition.
Cathode degradation is highly dependent on its composition and the interaction between the electrolyte and the cathode surface. The theoretical capacity of Li1-xCoO2 is as high as 280 mAh g-1. At 0 < x < 0.5 (< 4.2 V vs. Li /Li+). Increasing the charge cutoff voltage makes more Li ions available (x > 0.5), but reduces cycling stability.
This is attributed to the transition from the monoclinic phase to the hexagonal phase. In addition, the LCO is accompanied by a significant expansion (2.6%) along the c-axis. Induced stress inside LCO particles affects structural stability.
A more severe phase transition was detected in the NCM cathode. The delithiation reaction at high pressure can transform the NCM structure into disordered spinel or rock-salt phases, which have favorable thermodynamic advantages at high charging voltages, accompanied by oxygen release. At the same time, due to the similar ionic radii of Li+ and Ni2+, the Li/Ni atoms are arranged in a mixed arrangement.
These results improve the surface impedance and hinder Li+ reinsertion. Transition metals dissolve. During the delithiation process on the positive electrode, metal ions may be oxidized, promoting the decomposition and dissolution of the electrolyte into the electrolyte.
In addition, the erosion of the cathode material by HF can also lead to the dissolution of transition metals. These dissolved TM ions not only cause the capacity fading of the positive electrode, but also destroy the SEI layer on the negative electrode side to exponentially accelerate the failure of the battery. Taking NCM523 as an example, the Mn content in the SEI has a greater effect on the battery loss capacity than the Ni and Co content.
If one Mn2+ ion is added to the SEI, ~102 more Li+ are lost. Therefore, transition metal dissolution leads to battery loss capacity by affecting both positive and negative electrodes.
Mitigate lithium battery loss capacity
Progress and classification of MCL strategies based on anode and cathode Solutions that may compensate for irreversible Li battery loss have been proposed by researchers over the past few decades. Annotated bars on the tree show representative strategies in the historical development of MCL. With extensive efforts devoted to MCL strategies, several promising MCL approaches have emerged so far. Therefore, for a clear understanding of MCL, the content of the tree categorizes MCL methods such as artificial SEI, prelithiation, and addition of sacrificial agents.
Anode-based MCL strategy, while Honglong represents a cathode-based LIB-based MCL strategy. These detailed MCL methods are detailed in the following sections. The MCL strategy can be traced back to the 1960s when the positive electrode was explored.
Alkali metal intercalation AxMX2 (A = alkali metal, M = Nb, Ta, etc., X = S, Se, etc.) can be synthesized by (i) heating a mixture of host material and lithium metal at high temperature; (ii) two Halogen compounds are contacted with alkali metals in different solvents (eg ammonia, naphthalene (NaPh) and tetrahydrofuran (THF)); (iii) Chemical reactions between transition metal oxides or halides and alkali salts in an H2S atmosphere; (iv) Electrically induced metal intercalation into the host in the cell. These early attempts can be seen as the precursors to some MCL methods such as artificial SEI, electrochemical lithiation, etc.
Negative MCL method
Comparison of different methods for constructing artificial SEI. (a) Decomposition of Li-Naph/DME on hard carbon (HC) anode and formation of artificial SEI in LIB. (b) HC forms SEI on HC in Li-Bp/THF solution. (c) Energy diagram depicting the formation of SEI depending on the redox potential of the Li-BP complex (LAC stands for Li-arene complex). (d) Formation of artificial SEI on the P/C anode in Li-Bp/THF.
Since the formation of SEI is mainly responsible for the battery loss capacity of the anode, we define artificial SEI as an MCL method to alleviate the chemical reaction between the anode material and the reducing agent caused by the formation of SEI during the initial cycle. The constructed artificial SEI is more stable than the electrochemical in situ formed SEI, thus greatly improving the ICE of LIBs.
The ideal reducing reagent for the construction of artificial SEI should meet the following criteria: (i) Low redox potential to increase reaction rate and extent. (ii) Mild reaction kinetics for safe operation. (iii) Environmental stability suitable for industrial production. (iv) Low cost, guaranteed economic benefits. Among the earlier reported lithium-containing reagents, lithium dimethylamide (LiN(CH3)2) and triethyllithium borate (LiBH(C2H4)3), the SEI formed by n-butyllithium (n-BuLi) is in the usability , convenience and quality.
However, a thick and brittle SEI was found in n-butyllithium/hexane solution, and n-butyllithium is unstable in humid air, causing safety concerns. Therefore, more chemicals with better SEI forming ability were developed as potential reagents, such as lithium/ammonia solution, Li-NaPh, lithium benzophenone (Li-BzPh), 2,3-dichloro-lithium 4 , 5-dicyanobenzoquinone (Li-DDQ) and lithium iodide (LiI).
Considering the redox potential of chemical reagents, NaPh and BzPh anions are generally used in THF, while iodide and DDQ anions are used in acetonitrile. Further understanding of the SEI composition by X-ray photoelectron spectroscopy (XPS) in the 1990s facilitated the development of artificial SEIs.
With the vigorous development of various anodes, the SEI derived from traditional electrolyte decomposition cannot maintain the integrity of the electrodes. Therefore, constructing artificial SEI is considered to be one of the most effective strategies to alleviate the battery loss of active Li+. Most of the reported chemical solutions cannot form artificial SEI on graphite, mainly due to the incompatibility of graphite with the solvent. Molecular engineering of chemical reagents promises to solve these problems.
By introducing different numbers of electron-donating alkyl groups into the benzene ring, the reduction potential of the reagent can be reduced, while increasing the geometric configuration space and avoiding solvent co-intercalation during the formation of artificial SEI. Furthermore, some weakly solvated solvents (2-MeTHF, THF, etc.) form contact ion pairs between Li+ ions and anions.
Lee et al. applied Li-Bp in weakly solvating solvents 2-MeTHF and THF to stabilize the contact ion pair (Li+-Bp-), enabling preferential decomposition of anions, consolidating artificial SEI and effectively suppressing solvent co-intercalation. The environmental stability of chemical reagents and compositions requires special attention. The former is related to the redox potential, and the latter is related to the degree of reaction.
While an inert atmosphere can help alleviate such concerns, manufacturing costs will go up. Qu et al. constructed an artificial SEI on the P/C anode using a Li-Bp/THF solution impregnation process. The solution remaining on the surface of the negative electrode is used as a protective layer to protect the product from the influence of the surrounding air.
Representative prelithiation methods. (a) Lithiation of graphite by lithium vapor. (b) Schematic illustration of the prelithiation of porous carbon nanotubes. (c) Schematic illustration of the LixM/graphene anode. (d) Prelithiation of silicon microparticles by a hydrogen-driven reaction. Pre-lithiation can directly replenish/offset the lithium source consumed in the initial and subsequent cycles, and thus is one of the more popular strategies for MCL.
Over the past five years, there have been several good review/opinion articles devoted to pre-lithiation coverage that provide a good complement to this section. Unlike these literatures, this paper mainly focuses on the effect of prelithiation on CEs, the advantages and disadvantages of different prelithiation methods, and omits the detailed history and rationale of prelithiation.
Pre-lithiation as defined herein refers to the chemical pre-incorporation of Li ions into the electrode without the use of an electrolyte, which can be achieved by the reaction of the electrode with a lithium-containing reducing reagent (such as Li vapor, LiOH, LiH), such as thermal lithiation, alloy reaction and surface reduction. Simple chemical reactions during prelithiation facilitate the preparation of lithiated anodes.
Due to its high reactivity, the lithiated anode will be covered by SEI when in contact with air or electrolyte. However, the high reactivity of lithiated anodes makes this method difficult to perform in ambient environment, which is incompatible with industrial applications.
A series of LixM composites were synthesized by Cui et al. The LixM composites were then processed into graphene sheet-based self-supporting anodes. The outer graphene layer endows LixSi nanoparticles with better air stability in an atmosphere of 20-60% relative humidity.
Furthermore, a dispersion-strengthening mechanism is proposed to improve the air stability of prelithiated micro-Si particles, which is achieved by the formation of an amorphous carbon oxide matrix in a CO2 atmosphere.
Pre-lithiation of alloy anodes. (a) Comparison of volume changes of different anode materials after lithiation. The size of the area is proportional to the degree of expansion, as shown in the right column, and the number is the expansion rate (%). (b) SEM characterization of broken bubbles on the surface of Sn foil after cycling. (c) Roll-to-roll preparation of LixSn anode. (d) [email protected] composites were prepared by ball milling, with IM average induction melting and BM average ball milling. (e) One-pot synthesis of Li22Z5 alloy and Li22Z5-Li2O composite.
The Sn anode has high catalytic activity for the decomposition of the electrolyte, which can induce the formation of porous SEI films, thereby reducing the electrochemical performance of the battery. Reducing the OCV potential of Sn to <0.5 V by roll-to-roll mechanical lithiation can suppress its catalytic ability and favor the establishment of a compact SEI.
In order to maintain the structural integrity of Sn anodes, the introduction of carbon matrix seems to be effective, such as SnLi4.4/C hybrid nanoparticles. Alleviating the expansion stress leading to the main battery loss capacity significantly extends the lifetime of the Sn anode.
Cui et al. proposed a one-pot metallurgical process that uniformly embeds LixSi domains into a Li2O matrix, and extended this approach to group IV metals as a general technique .
Among the synthesized Li22Z5-Li2O composites (Z=Ge, Sn, etc.), the LixGe-Li2O composite exhibits the best air stability, thanks to the highest binding energy between Li and Ge. Special attention should be paid to the stress induced by prelithiation, which may lead to cracking of the alloy anode, isolating ionic and electronic conduction.
Pre-doping guest metal atoms (Mn and Si, etc.) can refine the grains, facilitate ion migration, and aid in stress relief during pre-lithiation. In addition, heat treatment is also effective in optimizing grain boundaries.
Thermal energy can induce recrystallization of dislocations and polyangulation of grain boundaries. The dense grain boundaries guide Li ions during the mechanical lithiation process to ensure the structural integrity of the alloy anode.
Prelithiation using a lithium source. (a) Schematic illustration of the lithiation method using four different lithium sources (SLMP, thick lithium ribbon, thin lithium foil, and thin lithium foil with pinholes). (b) NMR spectrum of graphitic lithiation using lithium foil. (c) Schematic illustration of Li ion and electron transfer with and without buffer layer. (d) Schematic diagram of the preparation of lithiated anodes with poly(vinyl butyral) and poly(methyl methacrylate) layers.
Lithium used in direct contact prelithiation mainly includes the following categories: (i) pure lithium metal source; (ii) stabilized lithium metal powder (SLMP). In 1990, Dahn et al. lithiated carbon anodes by contacting lithium sheets in the presence of organic solvents. The short-circuit transfers Li ions from Li metal to Li-free anode materials, the extent of which depends largely on the contact area and reaction time.
Compared the direct-contact prelithiation kinetics of graphite and HC anodes with four Li sources (SLMP, thick Li strip, thin Li foil, and thin Li foil with pinholes). The lithiation rate is positively related to the surface area of the lithium source and the concentration gradient of lithium ions, but not to the introduced pores.
Once the lithium source is selected, the impregnation time becomes a critical factor that needs to be optimized to avoid residual lithium metal on the surface of the lithiated anode. The commonly used thick lithium foil can speed up the lithiation reaction, but at the same time increase the production cost due to the low utilization rate.
Kinetics of the prelithiation reaction. (a) Initial three charge/discharge cycles of the lithiated SiO electrode; (b) Galvanostatic charging curves of lithiated electrodes with different lithiation times; (c and d) Changes in CEs for different lithiation times. (e) Schematic illustration of the effect of Si particle size on the lithiation behavior of Si/C anodes in LIBs.
The fast reaction rate between lithium metal and electrodes makes it difficult to analyze the kinetics of the lithiation reaction using in situ methods. NMR spectroscopy was used to explore the rate constant of the prelithiation reaction.
As the degree of graphite prelithiation increases, the rate constant of the lithiation reaction decreases, which is attributed to the fact that LiCx is closer to the potential of Li than pristine graphite. Therefore, insufficient driving force will leave part of the lithium source on the graphite surface, which leads to safety issues.
The lithiation time is crucial for obtaining a safe lithiated anode, which can be optimized according to ICE and pre-lithiation capacity. Large-scale operation is complicated by the high reactivity of lithium metal and lithiated anodes to ambient air.
Coating a protective layer on lithium metal suppresses its reactivity, enabling: (i) suppress side reactions; (ii) modulate lithiation rate; (iii) modulate Li+ diffusion; (iv) mitigate volume change. Carbon nanotubes, poly(vinyl butyral), and poly(methyl methacrylate) have been used as resistive layers to tune Li+ diffusion. In these coated electrodes, the reaction rate is essentially determined by the area resistance of the buffer layer.
Proper reaction duration, which can ensure high ICE and reduce residual lithium on the anode, is a necessary prerequisite for the possible commercialization of prelithiation. The potential lithiation mechanism between the anode and Li metal was analyzed. As the Si particle size decreases, Li ions tend to lithiate Si rather than C in Si/C composites.
For larger silicon particles, even if the reaction time is prolonged, the lithiation is not fully completed. Glow discharge mass spectrometry of the lithiated Si/C anode shows that the distribution of Li in the particle phase and electrode depth is spatially inhomogeneous.
The negative electrode was prelithiated using SLMP. (a) Voltage curves of SiO/graphite/C anodes prelithiated with different amounts of SLMP. (b) Cycling performance of SiO|| NCM cells with and without SLMP. (c) Si-CNT||LiNiCoAlO2 without prelithiation (top), with SLMP prelithiation (middle), and with pressure-activated SLMP prelithiation (bottom).
FMC Corporation developed SLMP in 2004 to reduce battery loss capacity. Due to the long stabilization time (4 h), the localized corrosion reaction of SLMP forms the SEI film on the anode material at a controllable rate.
SLMP can be processed in a dry room because the Li2CO3 layer on the surface stabilizes the Li metal core inside. Therefore, SLMP is superior to pure lithium in terms of cost, safety, and convenience for industrial large-scale applications. However, SLMP is incompatible with common solvents such as N-methylpyrrolidone (NMP), dimethylformamide (DMF), and dimethylacetamide (DMA), which are commonly associated with polyvinylidene fluoride (PVDF) The adhesive binds to the electrodes coated on the LIB.
On the contrary, most hydrocarbons and some ethers can be used as alternative solvents, but due to the low density of lithium, the SLMP dispersion is not uniform. To ensure the homogeneity of the SLMP dispersion during prelithiation, various solvents or binders were investigated, including poly(styrene-co-butadiene)rubber (SBR)/polystyrene(PS)-xylene solution, lithiated polyacrylic acid, and SLMP/toluene suspension.
As an effective prelithiator, SLMP is well suited for Li-free all-solid-state Li-ion batteries. Lee et al. constructed an ASSB consisting of a Si-Ti-Ni alloy anode and a FeS/S cathode, in which SLMP was pre-added on the anode side to provide a sufficient Li source for the first discharge process, and the ASSB had 225Wh kg-1.
Electrochemical lithiation can be accomplished by carrying out electrochemical reactions, typically in cells with electrolytes. Electrochemical lithiation can quantitatively compensate for Li battery loss according to the voltage-capacity curve, half-cell (paired with lithium as auxiliary electrode) or full-cell (paired with over-lithium cathode).
The former can be called ex-situ electrochemical lithiation, while the latter can be regarded as in-situ electrochemical lithiation. Half cells have to reassemble the electrochemically treated anode by disassembling the cell, which not only doubles the consumption of electrolyte, but also requires an inert atmosphere. Therefore, high cost and process complexity hinder scale-up.
Schematic diagram of electrochemical lithiation. (a) The overall electrochemical lithiation mechanism using Li3V2(PO4)3 as the starting material. (c) The voltage of the full cell consists of Li3V2(PO4)3 anode and hard carbon anode. (c) Scalable application of electrochemical lithiation in roll-to-roll processes.
By rational selection of overlithiated cathodes, electrochemical lithiated electrodes can be directly used in full cells. Li et al. designed a HC||Li5V2(PO4)3 full cell, in which the HC anode can be lithiated during the conversion of Li5V2(PO4)3 to Li3V2(PO4)3.
Similarly, Wang et al. utilized the first Li+ extraction step (1.5–3.6 V) of the Li3V2(PO4)3 cathode to lithiate the HC anode (Fig. 12b), which resulted in a new LixC||Li2V2(PO4)3 battery with Form (Eq. 2 and 3):
However, these results benefit from the multiple voltage plateaus of the cathode, which are ineffective for most commercial cathodes with a single Li+ insertion/extraction step. Electrochemical lithiation outside the battery promises to simplify the process in battery systems. However, the reaction rate is difficult to control. Cui et al. managed to electrochemically lithiate carbon-coated SiOx (c-SiOx) with lithium foils, and the lithiation rate was determined by external resistors with different resistances.
As shown in Figure, electrochemical lithiation was further applied to the roll-to-roll preparation process, indicating the potential for practical applications.
MCL method for LIB cathode
Overlithiation is essentially a chemical or electrochemical reaction that adds excess lithium to the positive electrode to counteract the battery loss capacity during initial cycling.
Excessive lithiation of the positive electrode. (a) shows various lithium insertion potentials for different electrodes. (b) Schematic illustration of the preparation of Fe/LiF/C nanocomposites by ball milling. (c) Possible lithium intercalation sites in the V6O13 structural unit. (d) Schematic illustration of the perlithiation process of high-nickel NCM. (e) The charge/discharge curves of the LMO for the first cycle (2.2-4.3 V) and subsequent cycles (3.5-4.3 V) in the half-cell system. (f) The first three charge-discharge curves of the α-Fe2O3||Li1.26VPOF battery. (g) First and subsequent charge/discharge curves of LNMO in a half-cell system (3.5–5 V).
The structure and composition of the cathode fundamentally determine the appropriate perlithiation method. For example, idest, n-BuLi, LiI and LiOH/tetraethylene glycol systems are used as perlithiators for spinel LiMn2O4 cathodes, of which the latter two are more suitable due to their milder reducing ability.
In addition, TEG and LiOH are insensitive to moisture and do not require an inert atmosphere to implement this technique. In addition, chemical ball milling has a good effect on the overlithiation of converted fluoride cathodes.
However, it is rarely used for overlithiation of intercalated cathodes because it destroys the crystal structure of the cathode. The amount of pre-intercalated Li ions in the cathode needs to be carefully controlled to prevent detrimental phase transitions or structural collapse, especially for high-capacity cathodes that can incorporate multiple Li ions per molar unit.
Sacrificial additives donate lithium ions to the battery through irreversible decomposition, and generally have the following characteristics:
(i) high quality and volumetric capacity, minimizing the use of additives; (ii) High irreversibility, avoiding Li+ re-insertion after the first charge; (iii) Appropriate voltage window to increase capacity; (iv) Compatibility with all components of the battery, such as active material, electrolyte and separator. (v) Environmental stability for survival during industrial production. The authors summarize typical additives used in LIBs and lithium-ion capacitors (LICs).
Due to the similarities between LIBs and LICs, most additives can be employed in both devices. Despite significant progress in research on lithium-containing additives, a number of headaches plague large-scale applications:
(i) Gas production. Anions such as azide (LiN3), square (Li2C4O4), oxalate (Li2C2O4), ketomalate (Li2C3O5), and diketosuccinate (Li2C4O6) are easily oxidized, producing undesirable gases during charging (N2, CO2 and CO). (ii) Low ionic conductivity. To achieve high utilization of Li+ ions in additives, high electrical conductivity is required. (iii) Low delithiation ability.
Some additives, such as Li2Mn2O4, Li2MoO3 and Li2CuO2, have specific capacities of 249, 250, 251, 252, 253 (< 300 mAh g-1). Therefore, in order to obtain higher capacity, excess additives must be added at the expense of energy density. (iv) Incompatible with the manufacturing process.
Environmental instability or incompatibility with NMP solvents hinders the application of some additives. Therefore, the problems involved in positive electrode additives are expected to develop novel additives. Integrated Fe nanodomains with Li2O and LiF into Fe/LiF/Li2O nanocomposites.
The LiF domains can protect the composites from CO2 and H2O, while Li2O provides delithiation capability. As a result, this additive provided 550 mAh g-1 and proved to be effective in LCO, LFP and NCM cathodes with only 4.8 wt% addition. However, products with low electronic conductivity after the conversion reaction may affect the kinetics of the cathode reaction.
Attempts have also been made to improve the environmental stability of Li2S, such as mixing with ketjen black or compounding with polyacrylonitrile. However, the incompatibility of these composites with NMP solvent remains unresolved.
The Li2O decomposition mechanism deserves further investigation, as the Li2O hybrid NCM cathode shows a larger contribution to the enhanced capacity than Li2O. Amine et al. revealed that the oxidation of Li2O during the first charge produces a series of LixOy species (eg (Li2O)+, LiO, Li2O2, (Li2O2)+, LiO2.(Li2O)+, O2).
These LixOy species accelerate the decomposition of the electrolyte, which eventually forms an interfacial film on the electrode. In addition, the shuttle behavior of LixOy was also detected. These two side reactions create additional capacity.
Compared with Li2O additives, commercially available Li2O2 has better environmental stability and compatibility with conventional blending processes. The NCM cathode catalyzes the decomposition of Li2O2 with a high capacity of 1109 mAh g-1, but there is still a large amount of O2 evolution.
Li3N has three disadvantages: high reactivity with moisture, incompatibility with NMP, and release of N2 during charging. Goodenough et al. deposited Li3N on the surface of the LCO cathode instead of a conventional mixing process, which avoided possible reactions between Li3N and NMP solvent.
Building a passivation layer on Li3N powder can effectively improve its stability in air. The results show that the Li2CO3 insulating layer can increase the decomposition potential of L3N from 0.44V to 4.2V. During the initial charging process, the Li2CO3-passivated L3N provides a high capacity of 800 mAh g-1 equivalent to the conversion of Li3N to Li2N intermediate to obtain 1 mol of Li ions.
An anti-perovskite structure of Li2OHCl was recently reported as a promising additive. The molecule can be electrolyzed at 3.3 or 4.0V, depending on the acidity of the electrolyte, producing O2, HCl, and Li ions. In a weakly acidic electrolyte, Li2OHCl is oxidized at a constant voltage of 3.3 V, while in a neutral electrolyte, Li2OHCl can achieve a delithiation capacity of 812 mAhg-1, making it a promising Li+ supplementary material.
Surface coating and ion doping are two representative cathode structural modification strategies to alleviate the battery loss capacity, especially the continuous battery loss capacity. Surface coatings mitigate battery loss capacity by retarding electrolyte decomposition, metal dissolution, oxygen loss, and structural collapse.
Some excellent reviews discuss potential mechanisms for electrochemical enhancement of surface coatings. The positive effect of the positive surface coating stems from the following reasons:
(i) a physical barrier preventing exposure of the cathode to the electrolyte; (ii) a scavenger of HF that attacks the positive electrode; (iii) An inert layer that suppresses the release of oxygen. In this section, the main focus is on the MCL effect. Surface films used to mitigate battery loss capacity mainly fall into the following categories: (i) oxide coatings (MgO, ZrO2, TiO2, Al2O3, P2O5, etc.); (ii) metal fluorides (CoF2, CaF2, AlF3, etc.); (iii) ) metal phosphates (Li3PO4, AlPO4, etc.); (iv) carbon films; (v) solid electrolytes (Li3.3PO3.8N0.24 (LiPON), Li6.375La3Zr1.375Nb0.625O12 (LLZNO), etc.
Some metal oxides HF can be scavenged, but water is produced, which further reacts with the LIPF6-based electrolyte to produce HF again. The continuous corrosion reaction makes it difficult for metal oxides to remain stable during long-term cycling. Metal fluorides can largely avoid this worsening problem due to their considerable chemical stability.
Reported for the first time that AlF3 coating can reduce the interfacial resistance while suppressing the Co dissolution of LCO. Similar enhancements are also achieved on NCM cathodes. Subsequent studies showed that doping with guest ions can improve the ionic conductivity of AlF3, thereby further improving the cathode electrochemical performance of the cladding coating.
The coating concentration and calcination temperature have a great influence on the electrochemical performance of the coated cathode. LCO with 1.0 wt% AlPO coating had the best cycling stability. Higher AlPO4 coating concentration can greatly reduce Co dissolution at the expense of reversible capacity. This is due to the inhibition of Li+ diffusion by the AlPO4 coating.
Furthermore, Higher annealing temperatures (600 and 700 °C) compared to lower annealing temperatures (400 °C) could further alleviate the battery loss capacity even at high cut-off voltages (4.6 V). This is due to the better crystallinity of AlPO4, which supports fast Li+ diffusion.
Polymers show potential to improve cathode stability by coating on secondary and primary NCM particles. However, limited lithium ion diffusion in the polymer coating may impair the electrochemical performance. In order to enhance the Li+ transfer kinetics in the surface coating, LI3PO4 functionalized polyarylethersulfone as a coating.
Li3PO4 facilitates Li+ transfer along the polymer backbone, while poly(arylene ether sulfone) maintains interfacial stability. The average CE of the coating NCM811 over 500 cycles was 99.96%.
Surface coating has been shown to be an effective MCL method, but the reversible capacity of the coated cathode is reduced due to the inactivity of the coating. In addition, the ionic conductivity and chemical stability of the coatings still need to be improved.
Binary coatings or defective coatings can achieve improved performance due to their synergistic effect, but samples cannot be synthesized without complexation. Solid-state electrolyte coating is a promising strategy to obtain good protection and high ionic conductivity, but it is still in its infancy.
It should be noted that although artificial SEI exhibits similarities with surface coatings. But their composition and function are fundamentally different:
(i) Artificial SEIs consist of multiple components of reducing agents. In contrast, surface coatings usually consist of a single component. (ii) Artificial SEI is used to alleviate the Li battery loss caused by SEI formation at the anode, while the surface coating is mainly used to stabilize the structure of the cathode. Ion doping is another structural modification method for MCLs. Intercalated ions can alleviate battery loss capacity through the following mechanisms:
(i) increase ion conduction by substitution; (ii) suppress Li/Ni mixing by preventing transition metal migration; (iii) reduce oxygen loss by strengthening metal-oxygen bonds. The initial ion doping is mainly single cation doping, including various monovalent ions and multivalent ions. In addition, anion doping, such as F-, SiO44-, SO42-, etc., has also been developed. However, these ion doping methods have limited improvement in cathode performance compared to surface coatings.
Different ions of double cation doping methods (Mg/Ti, Al/Ga, La/Al, Na/PO42−, Mg/PO42−, etc.) have better synergy. Despite the improved capacity retention of dual-ion-doped cathodes, the mechanism behind mixed-ion doping remains unclear. Although they effectively suppress the phase transition, this hinders the further generalization of the dual-ion doping method.
Mitigate the battery loss capacity of lithium-sulfur batteries
LSBs have the advantages of high energy density and low cost, but have serious problems in cathode and anode. Some of these issues include polysulfide dissolution and shuttling, lithium dendrites, etc., all of which pose challenges to the safety and stability of LSBs. One possible solution to overcome these problems is to use non-lithium anodes (eg, graphite, silicon, metal oxides) instead of lithium anodes.
Li-metal-free LSBs can be prepared in two states: a discharged state and a charged state. The former uses lithiated S positive electrodes (such as Li2S) and conventional negative electrodes (such as graphite, Si), while the latter uses lithiated S negative electrodes (such as LixSi, LiC6) and conventional S positive electrodes. In both cases, some serious challenges materialize:
(i) The low capacity of the graphite anode makes it technically difficult to balance the high capacity of the S cathode due to the excessive and thick active material loaded on the current collector; (ii) Alloy-type anodes with high lithium storage capacity are promising, but they are prone to volume expansion and electrode pulverization, resulting in low ICE and poor cycling stability; (iii) The formation of SEI at the negative electrode leads to a large battery loss of Li+; (iv) Spontaneous reactions between polysulfides and lithiated anodes should also be addressed; (v) The air sensitivity of lithiated sulfur cathodes inhibits their commercialization. To address these issues, the MCL strategy is crucial for developing Li metal-free LSBs.
MCL method for lithium-sulfur battery anode
MCL method for negative electrodes in LSBs. (a) Nafion shields Si from reacting with polysulfides. (b) Mechanisms of SEI formation at graphite electrodes in dilute and (c) concentrated electrolytes. (d) Schematic diagram of the SiO2 layer on the Si anode. The SEI/CEI layer, mainly from electrolyte decomposition, basically determines the stability of the LSB.
For the first time, Suo et al. adopted a high-concentration electrolyte (Li[CF3SO2)2N(LiTFSI) in 1,3-dioxolane (DOL) and DME]) strategy in the Li-S system, which effectively prevented the shuttling reaction and alleviated the capacity fading. Chen et al. transplanted this strategy into a graphite||Li2S full cell.
TFSI- ions form ion-pairs with Li+ to form dense TFSI-derived SEIs on the graphitic anode (Figures 15b and 15c). Localized high-concentration electrolytes can achieve similar results to those formed in high-concentration electrolytes.
MCL method for lithium-sulfur battery cathode
Li2S is a promising lithiated cathode due to its high theoretical capacity (1166mAh g-1) and low cost. However, Li2Sx (x = 1–8) has poor conductivity and high potential barrier, requiring a high activation potential of about 4V.
Due to the air sensitivity of Li2S in the discharged state, it is difficult to commercialize it, so Zheng et al. chose Li-Naph/DME solution and air-stabilized polyacrylonitrile-sulfur composite (S-PAN) as the discharge cathode. In S-PAN, polysulfide formation is also eliminated when S is chemically bonded to the conductive framework.
In addition, in order to reduce the initial battery loss capacity, the silicon anode was also subjected to pre-lithiation treatment. The energy density of the full cell composed of lithiated silicon anode and L2S-PAN cathode is 710Wh kg-1, and the ICE is 93.5%. This work combines the advantages of anode-based and cathode-based MCL approaches to form a complete LSB battery.
Mitigate LIC battery loss capacity
As a promising energy storage system, LICs combine the advantages of LIBs and supercapacitors to achieve high power density, high energy density, and long cycle life. Unlike LIBs, however, no electrodes can provide Li ions to counteract the irreversible battery loss capacity in LICs.
Therefore, the application of MCL strategy in LICs is crucial. The high energy density of LIBs guarantees its dominance in the portable energy storage market. However, the Li+ (de)intercalation mechanism limits their application in high-power domains.
Electrochemical double layer capacitors (EDLCs) with high power density can be complementary to LIBs. To integrate the advantages of EDLC and LIB into one system, LIC is proposed based on a hybrid LIC of battery-type electrodes and (pseudo)capacitive electrodes.
As shown in Figure, LIC has a wider voltage window than EDLC due to the low redox potential of the battery-type anode. Amatucci et al. are pioneers in assembling hybrid LICs consisting of Li4Ti5O12 anode and activated carbon (AC) cathode.
This LIC exhibits an energy density of over 20Wh kg-1 in the 1.5-3.0V operating window. In order to further improve the energy density of the Li4Ti5O12 anode, different types of graphite can be used instead. Current LICs usually consist of a graphite anode and an AC cathode, i.e. the two-carbon model.
Therefore, there is also an irreversible capacity associated with SEI formation in LICs. Compared to Li-ion batteries, there is no Li-containing electrode to compensate for the battery loss of active Li-ions on the negative electrode.
Therefore, MCL has become a prerequisite for obtaining LICs with long cycle life. Since the emergence of LICs in the early 2000s, many MCL approaches have been reported, including electrochemical lithiation, synthetic SEI, and sacrificial additives.MCL method in LICs. (a) Voltage profiles of EDLC and LIC. (b) Cation and anion conversion in LICs in different potential ranges. (c) Four-electrode structure and voltage map of lic(c) before and after prelithiation. (f) LIC system consisting of HC/SLMP anode and AC cathode. (g) Preparation of HC/SLMP electrodes.
Furthermore, the SLMP lithiated HC anode can be scaled up by a roll pressing process. The obtained LIC pouch cells have capacitances of 250F and 395F, which can achieve 22 Wh kg-1 and 31.5 Whkg-1, respectively. Just like sacrificial additives in LIBs, researchers report a number of chemical additives to counteract the Li in battery loss in LICs. These additives should also meet the requirements mentioned in Section 3.2.2. The reported additives can be classified into the following categories: (1) lithium oxide; (2) lithium nitride; (3) lithium sulfide; (4) organic lithium salts.
Summary and outlook
In order to better achieve the purpose of MCL, the advantages and disadvantages of each MCL strategy need to be considered in basic research and future commercialization: (i) Artificial SEI. Since the formation of SEI in the initial cycle consumes a large amount of Li ions, the construction of artificial SEI can significantly improve the ICE of LIB. Artificial SEI can be conveniently formed by the reaction of negative electrode with chemical reagents.
However, the application of chemical reagents is limited by their redox potential and high reactivity. Most of the reported chemistries are suitable for non-graphitizable carbon and silicon anodes, but not for graphite anodes because of their higher redox potentials than graphite, and their incompatibility with graphite.
The construction of artificial SEI on graphite anode requires rational adjustment of chemical reagents. Furthermore, the high reactivity of chemical reagents and reaction products requires an inert atmosphere to operate. Surface coatings such as metal oxides, typical of man-made SEIs, can help avoid these problems.
They effectively mitigate the large volume change of alloy anodes by virtue of their mechanical strength. (ii) Prelithiation. Additional lithium sources can be directly pre-deposited into the electrodes by a pre-lithiation method, which will counteract battery loss. Li metal and SLMP show efficacy in the prelithiation of C and Si anodes.
However, due to the high reactivity of lithium, the lithiation rate using lithium metal is faster, so it is difficult to determine the amount of pre-stored lithium. Although SLMP is less reactive, its incompatibility with commonly used solvents such as NMP hinders its industrial application.
(iii) Electrochemical lithiation. By tuning the electrode potential, electrochemical lithiation can precisely mitigate the battery loss capacity, which is usually performed in batteries. Operational complexity limits its large-scale production.
Therefore, simplifying the electrochemical lithiation process will greatly facilitate its further applications. (iv) Excessive lithiation. Over-lithiated cathodes can mitigate lithium ion battery loss in the anode by consuming additional lithium, which can be quantitatively added to the cathode by chemical or electrochemical methods. In addition, excess lithium does not produce inactive residues after delithiation.
However, overlithiation is not suitable for all cathodes. The application of overlithiation is limited to cathodes with multiple lithiation/delithiation steps (LMO and LNMO, etc.). (v) Sacrificial additives. Sacrificial additives release Li ions through decomposition reactions. The capacity of the additive can be conveniently determined by the amount added. In theory, this MCL strategy can be applied to any cathode.
However, the decomposition of additives produces inactive residues that reduce the gravimetric capacity and energy density of the battery. This situation is further exacerbated for additives with lower delithiation capabilities. In addition, the gases released by the decomposition reaction also require attention.
(vi) Structural modification. Surface coatings and ion doping, among others, aim to mitigate battery loss capacity by suppressing transition metal dissolution, phase transition, and electrolyte decomposition. Similar to sacrificial additives, these methods are applicable to all cathodes.
However, the mechanism behind the structural modification remains ambiguous. Further analysis is required to deepen the understanding of this MCL strategy. For commercial application of the MCL method, the following factors also need to be considered: (i) Balancing energy density and cost.
The energy density of LIBs has increased from 90 Wh kg-1 in the 1990s to 250 Wh kg-1 now, while the cost has increased from $1000 kWh-1 to $150 kWh-1. In order to be a strong replacement for gasoline vehicles, battery costs must be further reduced to closer to $100 kWh-1. Applying the MCL approach in LIBs inevitably increases cost, albeit a substantial increase in energy density, requiring a balance between battery cost and energy increase.
(ii) Large-scale production. The compatibility of the MCL method with existing manufacturing processes is a decisive factor for practical applications, involving operating environment, slurry mixing, electrode preparation, etc. However, some MCL methods are not suitable for industrial production, such as the construction of artificial SEI, and inert atmospheres are required for highly reactive chemical reagents and products.
(iii) Environmental and Safety Issues. As widely used power devices, LIBs should be environmentally friendly and safe for consumers. For example, the use of metallic lithium or SLMP pre-lithiated anodes can effectively reduce the battery loss capacity, although over-lithiated cathodes can be suitable, but the high reactivity of lithium increases safety risks.
The battery loss of Li ions is offset by providing additional capacity, but this may lead to Li deposition and Li dendrite growth. In addition to the battery loss capacity, the voltage loss also means that the discharge voltage of the cathode decreases throughout the cycle, which is a common phenomenon of Li-ion battery degradation. In addition, the battery loss of oxygen also reduces Ni, activating surface remodeling, the layered-to-spinel phase transition.
Therefore, suppressing the voltage battery loss is also the key to achieve high energy density. Indeed, the structural modifications discussed earlier show promise in addressing voltage battery loss. However, trial and error efforts are still required in this area. Due to the inevitable limitations of various MCL strategies, the development of future MCL strategies should consider the following points:
(i) Design a suitable electrolyte. The electrolyte as the “blood” in LIBs plays a decisive role in affecting its electrochemical performance, which is related to the electrode/electrolyte interface. A suitable electrolyte can establish good interfacial chemistry, leading to high CE and long lifetime of LIBs. Furthermore, electrolyte engineering is the most promising strategy compatible with industrial production. For related industry information, please refer to Top 10 lithium ion battery electrolyte company.
(ii) Design a new MCL strategy. The reported MCL methods are mainly based on electrodes, and although MCL is highly efficient, it is not compatible with the battery industry. Designing new approaches requires a combination of cost, convenience, and efficiency. Exploring MCL strategies in LSBs, LOBs, and LICs should focus on the respective interfacial stability and electrolyte compatibility, rather than just grafting the applied methods.