Fig. 1

Failure mechanisms and modification strategies for high-voltage LCO"

Fig. 2

(a) crystal structure and (b) band structure of LCO"

Fig. 3

(a) Illustration of various intermediate phases of LCO (O1 and O3: “O”represents octahedral coordination environment of Li+ and the number represents the number of slabs of transition metal-oxygen (TMO2) in a unit cell; H1-3: representing the structure comprises alternating O1 and O3 blocks; O3(II) and O3(I): represent the TMO2 stacking type are preserved from O3 phase while the lithium/vacancy ordering are different from each other)[31]; (b) Structural variations for LCO in different delithiated states (dQdV, dQ: derivative quantity of electric charge, dV : derivative voltage)[22]; (c) Calculation of Li x CoO2 crystal parameters in various states (triangle, circle, pentagonal, diamond and square represent O3(I), O3(I), O3, H1-3 and O1 phase, respectively)[27]"

Fig. 4

(a) The O3-LiCoO2 (blue) transforms to O1-Li(1-x)CoO2 (orange) through the simultaneously consecutive glide in the ab plane and interlayer expanding along the c axis of CoO6 slabs[33]; (b) O3-LiCoO2 with δ1 = 9.847°[33]; (c) Charged Li0.5CoO2 and discharged Li0.65CoO2 with δ = 7.426° and 5.761°, respectively[33]; (d) O1-LiCoO2 with δ5 = 0°[34]; (e) Schematic illustration of the step-like surface degradation of high-voltage LCO[34]; TEM images for LCO (f) the first charge to 4.65 V, (g) after 200 cycles[35]; The reciprocal lattice pattern viewed along [210] direction for LCO (h) the first charge to 4.65 V; (i) the first discharge to 3 V, and (j) after 200 cycles[35]"

Fig. 5

Degradation mechanism of LCO-electrolyte interface at high voltage; (a) Possible interface failures: microcracks, loss of active material, phase transition, cobalt dissolution, oxygen release, gas production, electrolyte decomposition, CEI formation, HF attack, resistance increase, etc.[36]; (b) Degradation cycle based on HF attack and water regeneration[43]; (c) Surficial oxygen release causing structural transformation and electrolyte decomposition[46]"

Fig. 6

Failure mechanism under high-voltage fast-charging conditions: (a)—(d) 3D and 2D Ni valence state distribution of (a), (c) particle and (b), (d) representative nanodomain for (a), (b) rod-NMC and (c), (d) gravel-NMC at charged state in the first cycle, The scale bars are 3 μm and 1 μm for(a), (c) and (b), (d), respectively[53]; Visualization of the particle detachment: (e) Selected slice in a NMC particle; (f) The 3D rendering of the segmentation results (left), the calculated distributions of relative local electrical resistance over the surface (medium), the same particles presented without the void phase (right); The scale bars are 10 μm in (e), (f)[55]; (g) Microtomography data of a piece of NMC electrode (left) and 3D rendering of the nano tomography data (right); (h)—(j) Three of the representative particles are highlighted in red, green, and blue; The same lateral slice on the particle layer (k) close to the top surface and (l) the bottom of ten-cycled electrode[58]; (m) Lithium composition maps of individual platelet particles at various SOC state during the second cycle[59]"

Fig. 7

TEM images of cathodes (a) before and after storing for (b) 1 month, (c) 3 months, and (d) 6 months; TEM images of anodes (e) before storage and after storing for (f) 1 month, (g) 3 months, and (h) 6 months at 65 ℃[60]; (i) Schematic illustration for the degradation mechanism of NCM811 electrode at elevated temperatures (80 ℃)[62]"

Table 1

Performance comparison of high-voltage fast-charging LCO modified with various dopants. EC∶ethylene carbonate, DEC∶diethyl carbonate, DMC∶dimethyl carbonate, EMC∶ethyl methyl carbonate, FEC∶fluoroethylene carbonate, VC∶vinylene carbonate"

Fig. 8

(a) Co k-edge XAFS after Fourier transform; (b) CoO4 plane and Co-O bond length of C-LCO (top) and P-LCO (bottom). Calculated PDOS and corresponding Co 3d: t2g and O 2p bandgap plots for (c) C-LCO and (d) P-LCO; (e) Diagram of electron orbital occupancy changes; (f) Co spin configuration differential plots of C-LCO and P-LCO; (g) Changes in the contents of C-LCO and (h) P-LCO in each phase around 4.6 V. Schematic diagram of the structure of the O3 phase and the H1-3 phase[81]; (i) Fine crystal structure of NH4VO3-treated LCOs (denoted as NV-30LCO) for the first cycle; (j) Differential electrochemical mass spectrometry of NV-30LCO and (k) blank LCOs (denoted Bare-LCO); (l) Schematic diagram of the calculated Bader charge and V-doping of CoO6/VO6[71]"

Fig. 9

Co valence mapping in (a) C-LCO and (b) Se-LCO in the 1st and 60th cycles at a charged state (4.62 V); (c), (d) HRTEM images of the superficial layer of (c) C-LCO and (d) Se-LCO at the 120th cycle (the inset shows the corresponding fast Fourier transform (FFT) patterns); 1st: the first cycle; (e) Schematic and lattice structure of (left) mobile O escaping from the charged LCO lattice and (right) Se substituting for Ov; (f) Diffusion coefficients of Li+ at the 120th cycle of C-LCO and Se-LCO, respectively[98]"

Fig. 10

(a) Formation energies of LCO and Mg-doped LCO with different doping sites; (b) Structural evolution illustration between LCO and LMCO; (c) STEM-HAADF and (d) STEM-ABF images of LMCO; (e) Path 1 belongs to the neighboring Li sites along a/b-axis direction, (g) Path 2 along the diagonal neighboring Li ions. The corresponding Li ions migration energy of LCO and LMCO; (f) Path 1 and (h) Path 2[99]; (i) Schematic diagram of the function of substituted nickel ions in the lithium vacancies, the “screen effect”[101]; (j) High-resolution STEM images of the layered structure at the center (left) and surface (right); (k) Schematic figure of the ion exchanging mechanism; (l) The rate capability at 0.1C charging and discharging at various current densities[67]"

Fig. 11

Schematic of crystal structure at the surface of (a) N@P and (b) Pristine (denoted as Pri); HAADF-STEM images of (c) N@P (left) and Pri (right); (d, e) The calculated Li diffusion coefficient obtained from GITT for Pri and N@P; (f) Rate capability of Pri and N@P; (g) Cycle performance of N@P in a pouch cell over 3.0—4.6 V[105]; (h) Raman spectra of pristine LCO and phosphidated LCO; (i) O 1s XPS spectra of phosphidated LCO; F 1s XPS spectra for (j) pristine LCO and (k) phosphidated LCO upon initial cycle[106]; (l) Schematic illustration of the synthesis of coherent spinel Li x Co2O4 and Li2SO4 co-coated and high-valence S gradiently doped LCO; (m) Atomic-resolution HAADF-STEM images of EG-LCO-20+0.1S; (n) Rate performance of EG@-LCO-20, EG-LCO-20 and EG-LCO-20+0.1S[104]"

Fig. 12

(a) Schematic structures of doped CoCO3, Co3O4 and D-LCO; The dQ/dV curves of (b) P-LCO and (c) D-LCO (the red arrows refers to various phase transitions); (d) The voltage profile corresponded contour plot of XRD pattern evolution of D-LCO (the black dash circles refers to O3(I)-O3(II) transition), the (003) peaks evolution of D-LCO and P-LCO; (e) The contour plot of (015) peaks evolution of D-LCO and P-LCO; (f) The voltage profile (black) and the cell volume evolution process (red) of D-LCO; (g) 3D spatial distributions and elemental distributions of Ti[74]; (h) Schematic of the differences in CEI between bare LCO and TMA-LCO. O K edge RIXS maps of (i) bare LCO and (j) TMA-LCO at 4.6 V charged state; (k) Optimized atomic structures of the Ti-doped slab in Li0.29CoO2[37]"

Table 2

Performance comparison of high-voltage fast-charging LCO modified with various coating materials"

Fig. 13

(a) Schematic diagram of ALCO with multilayer surface structure composed of LiAlO2 and LiCo x Al1-x O2; PDOS values of selected and total elements of (b) bare LCO and (c) ALCO. In-situ XRD plot and corresponding charge/discharge curve at 0.1C for (d) bare LCO and (e) ALCO; (f) The XRD plot at 4.7 V charged state for bare LCO and ALCO; (g) SEM images of bare LCO and (h) TEM images of ALCO after 200 cycles[139]; (i) Characterization and compositional analysis of F-LCO fluorinated interfacial structure; (j) cycling performance of F-LCO and P-LCO; (k) cycling performance of Graphite||F-LCO pouch cells[140]"

Fig. 14

(a) SEM images of LCO particles and carbon black in R-LCO; (b) Scheme of electrochemical indentation and crack formation in LCO crystals during charging R-LCO to a high voltage; (c) SEM images of CNT-cocooned LCO particles in CNT-LCO; (d) Scheme for preventing electrochemical indentation and crack formation in the LCO crystal when charging CNT-LCO to a high voltage; (e) Li concentration in the LCO particle in R-LCO; (f) the von Mises stress field of LCO in R-LCO; (g) Li concentration in the LCO particle in CNT-LCO; (h) the von Mises stress field of LCO in CNT-LCO; (i) Images of the free-standing CNT-LCO electrode before pressing. The inset shows the LCO particles in CNT-LCO; (j) Images of the free-standing CNT-LCO electrode after rolling pressing to a high density[153]"

Fig. 15

(a) The adsorb energy of the cathode and different ingredients at the interface of pristine LCO (P-LCO) and Y-LCO; Microstructures and related FFT patterns (inset) of different CEI for (b) P-LCO and (c) Y-LCO after 100 cycles; The energy diagram of the decomposition of LiPF6 to PF5 and LiF on the surface of (d) delithiated LCO (donated as CoO2) and (e) Y2O3; (f) The Ab initio molecular dynamics (AIMD) simulation of LiPF6 on Y2O3 with different times and (g) the related electron localization function[154]; (h) HAADF-STEM image and illustration of the interphase for AlZnO-LCO; (i) Electrochemical impedance spectra (EIS) for various electrodes measured before/after cycling[120]"

Fig. 16

(a) Schematic illustration of surface architecture and chemical potential profile μ of La3+ and Ca2+; δ denotes the oxygen off-stoichiometry; (b) Schematic of the atomic structure; (c) HAADF-STEM of La-LCO surface (after heat treatment) showing an architecture consisting of Region I, II and III, separated by the dashed lines, Scale bare 2 nm; (d) HAADF-STEM images and corresponding FFT patterns of Region I (up, perovskite La1-w Ca w CoO3-δ ) and III (down), Scale bar 5 nm; (e) PDOS of O 2p states and the local schematic structures of oxygen (insets) of LCO (left), unstrained LaCoO3 (medium), and -8% strained LaCoO3 (right). EF is the Fermi level, and solid lines are the average energy of the O 2pstates; (f) Characterization of the depth-dependent electrochemistry[156]; (g) Morphology of surface Li3PO4; (h) Surface, subsurface structures, and diffraction pattern analyses; (i) Schematic illustration of surface structure of SE-LCO; (j) Comparison of rate performances; (k) TEM image of the step-like surface degradation for P-LCO, and (l) the corresponded surface structures and related diffraction pattern analyses; (m) TEM image of the smooth surface for SE-LCO, and (n) the corresponded surface structures and related diffraction pattern analyses[33]"

Fig. 17

(a) Schematic diagram of the GDLCO particle with gradient Li/Co disorder structure; (b) The calculated energy as a function of the strain along the c axis for ordered and disordered samples at fully delithiated states; (c) HRTEM image for GDLCO; X-ray fluorescence spectroscopy (XRF) images (left) and spatial visualizations (right) of lattice expansion/contraction (Δd/d) for (d) LCO and (e) GDLCO[159]; (f) The 3D Co valence distribution in a single particle of P-LCO (left) and LCO (right) cathode; (g) Stress distribution of P-LCO and LCO cathode particles during the charging process using finite-element simulation[160]"

Fig. 18

(a) In-situ ATR-FTIR spectra of the LCO/electrolyte interface collected during initial charging; The current density is 25 mA/g; The spectrum collected at inflection voltage is colored in red. Schematic illustration of the electrolyte solvation configuration at the LCO/electrolyte interface during the Li+-solvation (charging) process at different voltages; (c) TOF-SIMS characterization of the 4.1 V and 4.2 V cycled LCO cathodes (below/above the inflection voltage) retrieved from the coin cells after 5 cycles. Normalized depth profiles of representative inorganic and organic fragments illustrate the CEI architecture; (d) Schematic illustration of CEI structure formed at charge cut-off voltages below/above inflection voltage[162]; Proposed functional mechanism of SPTF additive in carbonate electrolyte. HRTEM images of cycled LCO in base (e) and SPTF (f) electrolyte; (g) Electrochemical performance of LCO||graphite pouch cell[163]; (h) Schematic illustration of the mechanism on the stabilized 4.6 V LCO batteries; (i) Selected region of the (003) plane from the ex situ XRD patterns of LCO electrode cycled in FPE (up) and in TCE (down) during the 4th charging-discharging process at 0.2 C; (j) Long-term cycling performance of Gr||LCO coin cells obtained at 3C at 3—4.55 V (equivalent to 4.6 V vs. Li+/Li); (k) long-term cyclability of Gr||LCO pouch cells measured at 290 mA at 3—4.55 V[164]"

Fig. 19

(a) Representative functional groups of sulfone, sulfonate, and sulfate (left), and HOMO-LUMO energy levels of EC, SU, PS, DT, and MS (right); (b) The initial CE and (c) average CE of LCO-Li cells at different charge voltages (the charge cutoff voltages are 4.2, 4.4, and 4.6 V from left to right for every species); XPS spectra of (d) S 2p and (e) O 1s on the LCO surface at various charge voltages after 100 cycles; (f) The function mechanism of the MS additive for high-voltage LCO[165]; (g) Calculated HOMO and LUMO of different molecular; (h) The TEM images of LCO particles cycled in BE (up) and DE (down) after 50 cycles at 1C; (i) In-situ Raman spectra of the LCO in BE and DE at 0.5 mV/s upon the first cycle; (j) The selected Raman spectra at various charge/discharge states. TEM image of LCO particle after 2 cycles in (k) BE and (l) DE at 0.2 C; (m) The contents of transition metal Co ions in different electrolytes after 10, 50 and 100 cycles[166]"

Fig. 20

(a) TEM images and EDS mapping results of AP-LCO; (b) Evolution of Al-O-P signals in IR results upon 1st charging; (c) Evolution of the F 1s XPS results during cycles under 45 ℃; (d) Schematic diagram of chemical and morphological evolution of CEI on AP-LCO. Comparison of the temperature-dependent EIS spectra of (e) C-LCO and (f) AP-LCO; (g) Eavalues of C-LCO and AP-LCO in the 102th cycle under 45 ℃; (h) RCEI values in the 2nd and 102th cycle for AP-LCO under 45 ℃; (i) Comparison of the cycle stability curves at 1C in 3—4.6 V under 45 ℃ in Graphite||LCO cells[169]; (j) HRTEM image and corresponding FFT results of Z-LCO; (k) Cyclability at 1C, (l) Rate performance, and (m) GITT tests of LCO and Z-LCO between 3—4.6 V; (n) The schematic diagram for the densifying process of CEI layer for Z-LCO; O K-edge spectra of TEY mode from sXAS measurements of (o) LCO and (p) Z-LCO during 1st and 10th cycle[170]"

Fig. 21

In-situ XRD patterns of LCO electrodes with (a) PVDF and (b) FUC-PAM binder; (c, d) HRTEM images and fast Fourier transform (FFT) images (inset) of LCO electrodes with (c) PVDF and (d) FUC-PAM binder after 200 cycles; Co L-edge XAS patterns in TEY mode of LCO electrodes with (e) PVDF and (f) FUC-PAM binder; (g) Differential charge density of the LCO electrode with FUC-PAM binder; EIS spectra for electrodes at (h) pristine and (i) 100th cycle[173]"

Fig. 22

(a) SEM image of the electrode with 1%Cabot+0.3%CNTs; (b) EIS spectra; (c) Charge/discharge profiles of the first circle; (d) Cyclability for electrodes with various conductive agent[174]"

Fig. 23

(a) Cross-section SEM image of LCO, Super P, and SACNT film composite electrodes; (b) Rate performance[179]; (c) Cross-section SEM image of LCO-LT (low tortuosity); (c) Rate performance of LCO-LT and LCO-HT (high tortuosity) electrodes; (e) Illustration of the function mechanism for LCO-LT and LCO-HT electrodes[184]"

Fig. 24

Schematic illustration of (a) the data collection process of cRED, and the structural evolutions of (b) N-LCO, (c) H-LCO during charge[34] Visualizations of the internal isolated pores and interconnected crack network are highlighted (in cyan) within the reconstructed particle for (d) pristine material, and for material degraded at (e) 0.1 C and (f) 10 C[186]"