Unveiling Nickel Chemistry in Stabilizing High‐Voltage Cobalt‐Rich Cathodes for Lithium‐Ion Batteries

Abstract

A practical solution is presented to increase the stability of 4.45 V LiCoO 2 via high-temperature Ni doping, without adding any extra synthesis step or cost. How a putative uniform bulk doping with highly soluble elements can profoundly modify the surface chemistry and structural stability is identified from systematic chemical and microstructural analyses. This modification has an electronic origin, where surface-oxygen-loss induced Co reduction that favors the tetrahedral site and causes damaging spinel phase formation is replaced by Ni reduction that favors octahedral site and creates a better cation-mixed structure. The findings of this study point to previously unspecified surface effects on the electrochemical performance of battery electrode materials hidden behind an extensively practiced bulk doping strategy. The new understanding of complex surface chemistry is expected to help develop higherenergy-density cathode materials for rechargeable batteries.

A Surface Se‐Substituted LiCo[O2−δSeδ] Cathode with Ultrastable High‐Voltage Cycling in Pouch Full‐Cells

“…[ 3,4 ] However, the exact mechanism that caused the quick fading of high‐voltage LCO has not yet reached consensus. [ 5,6 ]…”

“…Continuous OL can be a killer problem to high‐voltage cycling. [ 10 ] First, OL causes irreversible phase transformations (CoO 2 →Co 3 O 4 ) [ 11 ] (Figure S2, Supporting Information). As Co 3 O 4 is a “bad” spinel with both octahedral and tetrahedral Co occupation that block Li + diffusion, [ 6 ] when the “densified” Co 3 O 4 grows thick enough to enclose all the LCO lattices, the percolating Li + diffusion can be terminated, causing dramatic impedance increase.…”

“…[ 10 ] First, OL causes irreversible phase transformations (CoO 2 →Co 3 O 4 ) [ 11 ] (Figure S2, Supporting Information). As Co 3 O 4 is a “bad” spinel with both octahedral and tetrahedral Co occupation that block Li + diffusion, [ 6 ] when the “densified” Co 3 O 4 grows thick enough to enclose all the LCO lattices, the percolating Li + diffusion can be terminated, causing dramatic impedance increase. Second, as widely reported, [ 12,13,14 ] the oxygen released from the cathode, including O 2 and O α − radicals, is highly oxidative, which could decompose the carbonate‐electrolyte quickly and produce a thick cathode‐electrolyte interface (CEI) that degrades battery cycling.…”

“…O 2 escape from the LCO particle must result in Co reduction, [ 19 ] where the reduced Co ions (Co 2+/3+ ) would migrate to the adjacent tetrahedral or octahedral sites, transforming the charged layered lattice to spinel (Co 3 O 4 ) phase. [ 11 ] This “bad” spinel is not only electrochemically inactive, but also blocks Li + diffusion in cycling. [ 6 ] In this work, we performed Co X‐ray absorption near edge structure (XANES) mapping at the National Synchrotron Light Source II (NSLS‐II) of Brookhaven National Laboratory, to track the Co valence distribution in the charged particles.…”

“…[ 11 ] This “bad” spinel is not only electrochemically inactive, but also blocks Li + diffusion in cycling. [ 6 ] In this work, we performed Co X‐ray absorption near edge structure (XANES) mapping at the National Synchrotron Light Source II (NSLS‐II) of Brookhaven National Laboratory, to track the Co valence distribution in the charged particles. As shown in Figure 2c,d, when first charged to 4.62 V, while most of the Co in C‐LCO was charged to ≈+4 valence (in green), there appeared a few patches comprising of low‐valent Co ions (≈+3, in red) at the surface, and more seriously, the charged C‐LCO particle was almost fully covered by reduced Co ions after 60 cycles.…”