Realizing the fast-charging capability of LIBs would further expand their practical application. However, the consumption of lithium ions in the electrolyte limits the fast-charging of LIBs, therefore making it essential to improve the transport of lithium ions from the cathode to the anode. Conventional carbonate electrolytes used in LIBs do not exhibit satisfactory fast-charging performance; thus, there is an urgent need to develop new and efficient fast-charging electrolyte for use in LIBs. In 2019, Du et al. compared the fast-charging performance of high energy density (NMC811/graphite) LIBs when different lithium salts were added to the electrolyte [36]. They found that compared with the traditional LiPF₆ salt, lithium bisfluorosulfonimide (LFSI) exhibited a higher conductivity and higher lithium-ion transference number. During fast-charging over 12 min, a battery containing LiPF₆ electrolyte reached the cut-off voltage in 4.2 min, much earlier than the 7.4 min of a battery containing LFSI. During the first 12-min charge cycle, the LFSI electrolyte showed a 13% increase in capacity, with higher capacity retention of at 87.7% after 500 cycles and less lithium plating compared with LiPF₆. Zhang et al. developed an advanced electrolyte with a high oxidation potential of over 4.9 V and used it in NMC811-based LIBs to achieve excellent cycling stability across a voltage range of 2.5–4.4 V at room temperature and 60 ℃, good rate capabilities under fast-charging and discharging up to a rate of 3 C and good low-temperature discharge performance down to −30 ℃ with a capacity retention of 85.6% at a rate of C/5 (Figs. 3a and b) [37]. These excellent properties are mainly attributable to ethylene carbonate forming an effective cathode-electrolyte interface (CEI) on the NMC811 cathode, which improves the high voltage stability of the electrolyte, and a thin but strong conductive layer, rich in inorganic components, which protects the graphite anode. Additionally, the enhanced CEI layer can effectively protect the nickel-rich NMC811 cathode from corrosion by the electrolyte and inhibit the occurrence of cracking during cycling. The enhancement of the stability of the cathode and anode interface allows LIBs to continue long-term cycling at both room and high temperature, and the increase in electrode overpotential is negligible, thereby allowing a high energy density to be maintained during operation. The results also indicate that it is not the electrolyte conductivity and viscosity but rather the interfacial layers on both the anode and cathode that govern the LIB performance during fast charging/ discharging and low-temperature discharge. Jiang et al. found that a localized high-concentration electrolyte consisting of 1.5 mol/L lithium bis(fluorosulfonyl)imide in dimethoxyethane with bis(2, 2, 2-trifluoroethyl) ether as the diluent enables the fast-charging of working LIBs [38]. A uniform and robust SEI can be achieved on graphite surface through the preferential decomposition of anions. The established SEI can significantly inhibit the co-intercalation of ether-based solvent into graphite, allowing highly reversible Li⁺ intercalation/de-intercalation to be achieved. Graphite/Li cells exhibited fast-charging potential of 340 mAh/g at 0.2 C and 220 mAh/g at 4 C, excellent cycling stability (85.5% initial capacity retention over 200 cycles at 4 C) and impressive low-temperature performance (90 mAh/g at rate of 0.1 C at −20 ℃) (Figs. 3c and d). Wang et al. proposed an economical room temperature inorganic hydrated molten salt (RTMS) electrolyte with a wide electrochemical stability window of 3.1 V [39]. Compared with organic fluorinated molten salt, RTMS was made up of lithium hydroxide and sodium nitrate and was much lower in cost. Based on the RTMS electrolyte, a lithium/sodium ion hybrid battery was assembled using cobalt hexacyanoferrate (NaCoHCF) as the positive electrode and perylene-3, 4, 9, 10-tetracarboxylic acid dihydride (PTCDA) as the anode. The full-cell with the RTMS electrolyte exhibits excellent performance, exhibiting a high capacity of 139 mAh/g at 1 C, 90 mAh/g at 20 C and capacity retention of 94.7% over 500 cycles at 3 C (Figs. 3e and f). This excellent performances can be attributed to the unique properties of RTMS, such as its wide electrochemical window, being solvated water-free, and its high Li⁺ ion mobility, which promotes the excellent Li-ions insertion and extraction capacity of NaCoHCF. Zou et al. proposed a new carbonate-based high-voltage electrolyte using a mixture of ethyl methyl carbonate (EMC) and methyl acetate (MA) [40]. At a normal concentration of 1.2 mol/L LiPF₆, the electrolyte exhibited high stability at high voltage (4.5 V vs. Li/Li⁺), lithium-dendrite-free features upon fast-charging (162 mAh/g at 3 C) and superior long-term battery performance at low temperature. More importantly, a pioneering interface model related to the Li⁺ solvation structure was proposed for the cathode and anode, revealing the molecular-scale interactions of Li⁺ solvent anions on the electrode surface and the role they play in battery performance.

There has been heavy development of new systems, such as high-concentrated, local high-concentration and perfluorinated electrolytes. The improvement in battery performance when using these new electrolytes can be mainly attributed to the formation of a strong CEI film, which helps inhibit electrolyte decomposition. However, the molecular-scale interactions of Li⁺ ions, anions and solvents at the electrolyte-electrode interface are not yet fully understood; and the influence that they have on Li⁺ ion transport, the electrochemical stability of the electrolyte and electrode performance is also unclear.

When LIBs are rapidly charged, various polarizations, such as the resistance between the electrode/electrolyte, limit the utilization of active materials and increase the growth of lithium dendrites [13]. Therefore, it is necessary to design a chemically stable ion conductive passivation layer to prevent further electrolyte decomposition of the electrode [41]. A good way of forming a stable passivation layer is to add a small amount of foreign molecules (also known as additives) to the bulk electrolyte [42]. Therefore, in the development of fast-charging LIBs, it is particularly important to develop efficient electrolyte additives.

In 2018, Son et al. used the reducing compounds ethylene carbonate, fluoroethylene carbonate and ethylene carbonate to conduct a comparative study to evaluate the fast-charging of a LiNi_0.6Co_0.2Mn_0.2O₂ (NCM)/graphite full battery ability [43]. This investigation revealed that the combination of fluoroethylene carbonate and dimethyl carbonate in the electrolyte leads to fast-charging NCM/graphite full cells that show the excellent capacity retention of 79% after 1000 cycles at a high charging current density of 6 mA/cm², corresponding to 2 C and a discharge rate of 1 C without the occurrence of Li plating on the graphite anode (Figs. 4a and b). This excellent fast-charging performance can be mainly attributed to the formation of a low-resistance interface layer on the graphite anode and the effective dissolution of lithium ions at the NCM cathode. Shi et al. used fluorosulfonyl isocyanate (FI) as an electrolyte additive to create a graphite anode SEI [33]. Due to its high reduction potential of > 2.8 V vs. Li⁺/Li, FI is reduced before the carbonate-based electrolyte, leading to the formation of a conductive SEI on the surface of the graphite. This SEI serves as a thick and protective inorganic inner layer that prevents the growth of an outer organic layer and has the effect of greatly reducing the resistance of the graphite/electrolyte interface. Therefore, compared with the reference liquid electrolyte (LP30), Li/graphite cells with 2 wt% FI exhibit excellent rate performance at room (20 ℃) and low (0 ℃ and −20 ℃) temperature (Figs. 4c and d). Han et al. reported an electrolyte additive, (trimethylsilyl)isothiocyanate (TMSNCS) based on aminosilane, which has good electron-donating ability and can scavenge HF and PF5 [44]. TMSNCS effectively suppresses the formation of the interfacial layer on the anode and cathode in LiPF₆-based electrolyte, leading to the interfacial layer exhibiting long-term stability. After 300 cycles at a charge rate of 2 C and a discharge rate of 1 C, NCM622/graphite full-cell with 0.1% TMSNCS delivered a superior discharge capacity of 144 mAh/g, exhibiting excellent capacity retention of 91.8% (Figs. 4e and f). Hekmatfar et al. reported the effects that the electrolyte additives fluoroethylene carbonate, ethylene carbonate and propane sulfone have on a LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) electrode and lithium (half-cell) and graphite (full-cell) battery, and investigated the effect on performance when the electrodes were combined [45]. Using X-ray photoelectron spectroscopy and scanning electron microscopy, the CEI layer that formed on the NMC532 electrode upon cycling at 4.5 V was studied. In this way, the electrochemical performance of the electrode was found to be related to the chemical composition, thickness and morphology of the CEI. All the electrolyte additives studied showed beneficial effects in both half- and full-cell systems, confirming the formation of an effective passivation layer and the protection of the electrolyte from further decomposition under high voltage. It was found that the thickness of the CEI layer formed on the NMC532 electrode is different in the half- and full-cell configurations. Studies have shown that ethylene carbonate and propane sulfone have a positive effect on the formation of the CEI and are the best additives to use to improve battery performance. The SEI that formed on the graphite electrode was also investigated and compared with the CEI layer formed on the cathode. The detection of typical SEI reduction products on the surface of the cathode confirmed the occurrence of cross-talking between the two electrodes. Zhao et al. reported a simple approach by which to prepare a lithium difluorophosphate (LiPO₂F₂) solution as an effective film-forming additive via the direct adding of Li₂CO₃ into a solution of LiPF₆ at 45 ℃ [46]. Benefitting from the significantly reduced interface resistance (RSEI) and charge transfer impedance (R_ct) of both the cathode and anode upon the addition of the prepared LiPO₂F₂ solution into a baseline electrolyte, the cycling performance of the graphite||NMC532 pouch cell was remarkably improved under all of the tested condition (665 mAh/g after 350 cycles, representing 59% of its initial capacity at a rate of 0.2 C rate at −25 ℃, and 1127 mAh/g after 350 cycles, repersenting 69.3% of its initial capacity at 45 ℃). Cheng et al. added lithium bis(oxalate)borate (LiBOB) and dopamine (DA), which have the highest-occupied molecular orbital energy levels, as functional additives to the traditional carbonated electrolyte [47]. The results showed that LiBOB and DA form a strong N, B, O-rich inorganic/polymer CEI on the LiNi_0.8Co_0.1Mn_0.1O₂ electrode. The CEI film not only eliminates the possibility of adverse reactions occurring at the CEI and prevents the electrolyte from penetrating into the grain boundary, but also prevents the formation of inactive rock salt phases on the surface of the material. More importantly, the N, B, O-rich interface layer enables a fast Li⁺ diffusion kinetics process to ensure the high-rate performance of the cathode. Benefitting from the synergistic effect of dual additives containing LiBOB and DA, the cell exhibits high capacity retention of over 92% after 200 cycles at a rate of 1 C and also exhibits a high specific capacity of 118 mAh/g at a high rate of 20 C.

The above studies show that the introduction of additives into the electrolyte can lead to the in situ formation of a CEI, thereby reducing or even inhibiting the electrolyte reactions that take place at the electrode interface under high pressure. However, successful electrolyte design requires a detailed study of the performance of the SEI and CEI under different cycling conditions, especially the upper cut-off potential.

In standard LIBs, organic carbonate-based non-aqueous aprotic electrolytes are used. Although being the most advanced electrolytes, organic carbonates also have some disadvantages, such as their moderate ionic conductivity and low flashpoint [42, 48-50]. To further improve the level of battery electrolytes, many solvent classes have been comprehensively investigated to replace organic carbonates [42, 51-56]. Among them, nitriles and other cyano compounds exhibit high ionic conductivity and low-temperature cycling performance [57-64] and are thus expected to play an important role in the field of fast charging of LIBs. Hilbig et al. adjusted the electrolyte formula of nitrile: ethylene carbonate: fluoroethylene carbonate (BN: EC: FEC) solvent/cosolvent/additives and obtained the electrolyte formula of 1 mol/L LiPF₆ in nitrile: ethylene carbonate (9:1) + 3% fluoroethylene carbonate, which was evaluated in terms of its ionic conductivity, electrochemical stability window, and constant current cycling performance in NMC/graphite batteries [65]. At low temperatures of around 0 ℃, the conductivity of the nitrile: ethylene carbonate-based electrolytes was at least 32% higher (7.69 mS/cm) compared to the nitrile: ethylene carbonate-based and the organic carbonate-based electrolyte (5.83 mS/cm) electrolytes. A LIBs assembled using a nitrile-based electrolyte exhibited an average specific discharge capacity at 5 C of 103 mAh/g, which is nearly twice the capacity of the cell assembled using the reference electrolyte (66 mAh/g). Furthermore, a C-rate evaluation was performed up to 20 C.

In summary, the use of low-viscosity co-solvents, high-concentration electrolytes and additives that can achieve ideal SEI performance for fast-charging is an effective strategy to develop LIBs with a high charging rate. It should be emphasised that the rate, capacity and life of electrolytes require careful evaluation. Meanwhile, the rate capabilities of modified materials or preparation methods, including those of electrodes and electrolytes, have been mainly explored on the laboratory scale, which may not always translate into commercial batteries that show the corresponding improvements. Therefore, multidisciplinary efforts are indispensable in realizing the rapid charging of LIBs.