Dynamic behaviour of interphases and its implication on high-energy-density cathode materials in lithium-ion batteries

Undesired electrode–electrolyte interactions prevent the use of many high-energy-density cathode materials in practical lithium-ion batteries. Efforts to address their limited service life have predominantly focused on the active electrode materials and electrolytes. Here an advanced three-dimensional chemical and imaging analysis on a model material, the nickel-rich layered lithium transition-metal oxide, reveals the dynamic behaviour of cathode interphases driven by conductive carbon additives (carbon black) in a common nonaqueous electrolyte. Region-of-interest sensitive secondary-ion mass spectrometry shows that a cathode-electrolyte interphase, initially formed on carbon black with no electrochemical bias applied, readily passivates the cathode particles through mutual exchange of surface species. By tuning the interphase thickness, we demonstrate its robustness in suppressing the deterioration of the electrode/electrolyte interface during high-voltage cell operation. Our results provide insights on the formation and evolution of cathode interphases, facilitating development of in situ surface protection on high-energy-density cathode materials in lithium-based batteries.

, C 2 P -, C 3 O 2 F -, and C 5 OFfrom left to right. These species, along with C 2 Fand C 3 OFin the main article, collectively represent the spontaneously formed CEI due to the reactivity between carbon black and the electrolyte. All spectra are normalized by ROI coverage and drawn to the same scale in each panel. Note that a fair amount of CH 2 and C 3 H 2 are detected on the pristine electrode due to inevitable sample contamination. The migration of CEI species from the carbon/binder towards the active material is clearly shown. Figure 8: SEM images of the as-prepared samples used in the current study:

Supplementary
transition-metal hydroxide precursors, Ni 0.7 Co 0.15 Mn 0.15 (OH) 2 , with an overall spherical shape and narrow particle-size distributions of 8-10 µm (a), 12-14 µm (b) and 18-20 µm (c), and   which are integrated over 600 s of Cs + sputtering with 10 s interval and 2 scans per step. It can be seen that the CEI species in part decompose during the high-voltage battery operation, while the active mass dissolution is severely aggravated.
(500 s of Cs + sputtering) for the 8-10 µm (a), 12-14 µm (b), and 18-20 µm (c) composite electrodes after 3 cycles (data were collected separately at different locations on the composite electrodes and one is shown here; the same applies to Supplementary Fig. 21 and 22). The CEI formation depth, upon cell operation, is still larger with the increasing secondary particle size. material. The images show that the primary particle surface of the pristine sample is free of the electrochemically generated rock-salt phase (NiO); only a thin layer (~ 2 nm) of the cation-mixing phase is present at the surface. Figure 29: High-resolution transmission electron microscopy images of the pristine (left) and cycled (right) LiNi 0.7 Co 0.15 Mn 0.15 O 2 primary particle surface. Clearly, after 100 cycles at room temperature, the rock-salt phase becomes thick (more than 50 nm) along the lithium diffusion channels in the layered host lattice. It can also been seen that the rock-salt phase is porous, compared to the pristine material.  Polycarbonates CH2 -, C2HO -C3H2 -, CH3O -One possible route for the formation of these compounds are believed to be the (oxidative) polymerization of cyclic carbonates in electrolytes.
Fluorinated organic species are usually generated when carbon blacks or carbonate solvents in electrolyte solutions react with HF.
LiF, LixPOyFz C2P -, 7 LiF2 -, PO2 -, POF2 -Commonly found in electrolyte solutions that employ fluorinated salts such as LiPF6 and LiBF4. Despite being a salt decomposition product, LiF can also be formed from reactions between semicarbonates/cathode-materials and HF.
Li2O, Li2CO3 7 LiO -, 7 LiO2 -Readily formed in native surface film during material synthesis/storage, characteristic on transition metal oxides. Li2CO3 is generated when Li2O reacts with moisture and CO2 in ambient environment.
NiO 58 Ni -, 58 NiO -It is present on the surface of nickel-rich layered oxides upon electrochemical implementation. At the highly delithiated stage, the tetravalent Ni ions tend to migrate to the neighboring vacant Li sites during which the original layered R3 ̅ m structural configuration transforms into the rock-salt Fm3 ̅ m phase. Table 2: Common components present in the interphases spontaneously and electrochemically formed on layered high-energy cathode oxides upon electrochemical operation.

Supplementary
The irreversibly formed rock-salt phase (NiO) is also included. In this study, we particularly focus on the fluorinated organic species (major components in CEI) and metal fluorides (major active mass dissolution products) in the cathode interphases.
C 3 OF -) for interphasial species on cathode particle surface migrated from carbon black prior to electrochemical operation. Data were collected at multiple locations on the composite electrodes (mostly 5) and this table summarizes those for the 8-10 µm LiNi 0.7 Co 0.15 Mn 0.15 O 2 electrodes (with ROI-1 applied). The standard deviation is obtained by: Subsequently, all obtained values are doubled to yield a reasonable estimate of the thickness of the CEI. For example, for the 8-10 µm LiNi 0.7 Co 0.15 Mn 0.15 O 2 after 20 cycles, the CEI formation depth obtained here is ~ 202 ± 22 seconds (which translates to ~ 5.5 -6.5 nm).