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Near-surface reconstruction in Ni-rich layered cathodes for high-performance lithium-ion batteries

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Hoon-Hee Ryu
Hoon-Hee Ryu
Hoon-Hee Ryu
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Hyung-Woo Lim
Hyung-Woo Lim
Hyung-Woo Lim
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Sin Gyu Lee
Sin Gyu Lee
Sin Gyu Lee
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Yang-Kook Sun at Hanyang University
Yang-Kook Sun
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Abstract and Figures

The instability of the Ni-rich layered cathode materials in lithium-ion batteries is attributed to their labile surface reactivity. This reactivity induces the formation of residual lithium impurities on the cathode surface and severe side reactions with the electrolyte. Here we propose a washing process using Co-dissolved water for simultaneously removing residual lithium and forming a protective coating on Ni-rich layered cathodes. The washing induces the reconstruction of the near-surface structure through reactions with the residual lithium compounds, thereby preventing direct contact between the electrolyte and the Ni-rich surface. An additional fluorine coating on the washed cathode impedes the decomposition of salts, preventing the by-products from triggering autocatalytic side reactions at the electrolyte–cathode interface and thereby suppressing gas generation during cycling. The combination of these near-surface reconstructions synergistically extends the cycle lives of Ni-rich cathodes and satisfies the requirements concerning energy density, durability and safety for next-generation batteries in practical applications.
Formation of Co-rich layer on the cathode surface a, TG–MS analysis of the unwashed cathode, unwashed cathode mixed with 1 mol% of NH4F, the cathodes rinsed with Co-dissolved solution followed by vacuum drying and heat-treated Co-washed cathode mixed with 1 mol% of NH4F: temperature profile, relative mass changes, derivative mass and observed H2O and CO2 evolution during the heating. Thermal induced reactions mostly occurred in the unshaded regions of (1) and (2) ranging below 120 °C and between 150 and 450 °C, respectively. b–d, TEM–EDS maps (b) and HAADF STEM (c,d) with Fourier-filtered images of the F–Co-washed cathode surface at the marked regions in panel b by yellow marks, respectively. Source data
Formation of Co-rich layer on the cathode surface a, TG–MS analysis of the unwashed cathode, unwashed cathode mixed with 1 mol% of NH4F, the cathodes rinsed with Co-dissolved solution followed by vacuum drying and heat-treated Co-washed cathode mixed with 1 mol% of NH4F: temperature profile, relative mass changes, derivative mass and observed H2O and CO2 evolution during the heating. Thermal induced reactions mostly occurred in the unshaded regions of (1) and (2) ranging below 120 °C and between 150 and 450 °C, respectively. b–d, TEM–EDS maps (b) and HAADF STEM (c,d) with Fourier-filtered images of the F–Co-washed cathode surface at the marked regions in panel b by yellow marks, respectively. Source data
… 
Characterizations of the reconstructed near-surface properties a,b, A TEM image and the corresponding EDS overlay map showing Co and F distribution (a) and EDS line scan results of the surface of the F–Co-washed cathode (b) along a green arrow in panel a. The vertical dashed lines in panel b indicate the thickness of each coating layer. c, Comparison of F 1s, O 1s and Li 1s XPS spectra of the unwashed, F-coated, deionized water-washed (DI-washed), Co-washed and F–Co-washed cathodes. The vertical dashed lines in panel c indicate the binding energy of XPS peaks for each cathode, and the dashed curves indicate the fitted profiles by peaks from the residual lithium compounds and intercalated lithium on the surface. d, Gas evolution from pouches featuring each 4.3 V charged cathode with electrolyte during storage at 60 °C. In panel d, black filled-square data represent the volume changes of the pouch containing electrolyte and uncharged unwashed cathode. Source data
Characterizations of the reconstructed near-surface properties a,b, A TEM image and the corresponding EDS overlay map showing Co and F distribution (a) and EDS line scan results of the surface of the F–Co-washed cathode (b) along a green arrow in panel a. The vertical dashed lines in panel b indicate the thickness of each coating layer. c, Comparison of F 1s, O 1s and Li 1s XPS spectra of the unwashed, F-coated, deionized water-washed (DI-washed), Co-washed and F–Co-washed cathodes. The vertical dashed lines in panel c indicate the binding energy of XPS peaks for each cathode, and the dashed curves indicate the fitted profiles by peaks from the residual lithium compounds and intercalated lithium on the surface. d, Gas evolution from pouches featuring each 4.3 V charged cathode with electrolyte during storage at 60 °C. In panel d, black filled-square data represent the volume changes of the pouch containing electrolyte and uncharged unwashed cathode. Source data
… 
Comparison of electrochemical performances depending on surface modifications a,b, Initial charge–discharge curves at 0.1 C (a) and 0.5 C cycling (b) using coin-type half cells. c, Long-term cycling performance using pouch-type full cells. Source data
Comparison of electrochemical performances depending on surface modifications a,b, Initial charge–discharge curves at 0.1 C (a) and 0.5 C cycling (b) using coin-type half cells. c, Long-term cycling performance using pouch-type full cells. Source data
… 
Post-mortem analyses of the cycled cathode a–f, TEM images with corresponding EDS maps (a,c,e) and HAADF STEM images (b,d,f) with Fourier-filtered images of the marked region for the cycled F–Co-washed, Co-washed cathodes after 4,000 cycles and the cycled DI-washed cathode after 3,000 cycles, respectively. The yellow dashed lines in panels b, d and f indicate the border line of the damaged structure from the surface. g–i, Contour plot of the (003) reflection measured via operando XRD for the cycled F–Co-washed (g), Co-washed (h) and DI-washed cathodes (i).
Post-mortem analyses of the cycled cathode a–f, TEM images with corresponding EDS maps (a,c,e) and HAADF STEM images (b,d,f) with Fourier-filtered images of the marked region for the cycled F–Co-washed, Co-washed cathodes after 4,000 cycles and the cycled DI-washed cathode after 3,000 cycles, respectively. The yellow dashed lines in panels b, d and f indicate the border line of the damaged structure from the surface. g–i, Contour plot of the (003) reflection measured via operando XRD for the cycled F–Co-washed (g), Co-washed (h) and DI-washed cathodes (i).
… 
Investigation of the preformed LiF effect on the cathode surface through XPS depth analysis a–c, Comparison of XPS depth profiles (a) profiles after etching (b) and profile deconvolutions (c) in the F 1s region for the cycled unwashed and F-coated cathodes after 3,000 cycles and Co-washed and F–Co-washed cathodes after 4,000 cycles. The vertical dashed lines in panel c indicate the peak positions corresponding to PVdF, LixPFyOz, and LiF, respectively, and dashed curves in panel c indicate the combined profiles with the three peaks. Source data
+1
Investigation of the preformed LiF effect on the cathode surface through XPS depth analysis a–c, Comparison of XPS depth profiles (a) profiles after etching (b) and profile deconvolutions (c) in the F 1s region for the cycled unwashed and F-coated cathodes after 3,000 cycles and Co-washed and F–Co-washed cathodes after 4,000 cycles. The vertical dashed lines in panel c indicate the peak positions corresponding to PVdF, LixPFyOz, and LiF, respectively, and dashed curves in panel c indicate the combined profiles with the three peaks. Source data
… 
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Nature Energy | Volume 9 | January 2024 | 47–56 47
nature energy
https://doi.org/10.1038/s41560-023-01403-8
Article
Near-surface reconstruction in Ni-rich
layered cathodes for high-performance
lithium-ion batteries
Hoon-Hee Ryu   1, Hyung-Woo Lim   1, Sin Gyu Lee   1 & Yang-Kook Sun   1,2
The instability of the Ni-rich layered cathode materials in lithium-ion
batteries is attributed to their labile surface reactivity. This reactivity
induces the formation of residual lithium impurities on the cathode surface
and severe side reactions with the electrolyte. Here we propose a washing
process using Co-dissolved water for simultaneously removing residual
lithium and forming a protective coating on Ni-rich layered cathodes.
The washing induces the reconstruction of the near-surface structure
through reactions with the residual lithium compounds, thereby preventing
direct contact between the electrolyte and the Ni-rich surface. An additional
uorine coating on the washed cathode impedes the decomposition of salts,
preventing the by-products from triggering autocatalytic side reactions at
the electrolyte–cathode interface and thereby suppressing gas generation
during cycling. The combination of these near-surface reconstructions
synergistically extends the cycle lives of Ni-rich cathodes and satises
the requirements concerning energy density, durability and safety for
next-generation batteries in practical applications.
Being pursued as a way to tackle climate change, the electrification of
transportation is one of the most evident changes that one can expe-
rience in daily life. Major countries have enacted policies to support
climate ambitions in the electric vehicle (EV) market, and global EV sales
continuously achieve new records, exceeding 10 million in 2022 (ref. 1).
This rapid expansion of the EV market has emphasized the importance
of lithium-ion batteries (LIBs), as the performances and costs of EVs
are significantly influenced by them. The competition with internal
combustion engine vehicles in terms of the driving range has led to
a rapidly growing demand for high energy density LIBs using Ni-rich
layered cathodes.
Ni-rich layered cathodes provide high energy densities but suffer
from rapid capacity fading. The instability of Ni-rich layered cathodes
is attributed to side reactions originating from the labile Ni4+ ions
at the charged cathode surface, such as electrolyte decomposition,
metal dissolution and oxygen evolution
2
. Microcracking is a main fac-
tor inducing rapid capacity fading because it exponentially increases
the interface area between the cathode and electrolyte3,4. In addition
to electrochemical issues, the surface instability of Ni-rich layered
cathodes results in inevitable chemical reactions to form the resid-
ual lithium compounds at the cathode surface. The residual lithium,
formed by even short-term exposure to the ambient air and moisture,
decomposes to produce HF and gaseous species, posing potential
safety risks to LIB usage
5–10
. In addition, the presence of the residual
lithium compounds increases the difficulty of the manufacturing pro-
cess owing to the gelation of the cathode slurry and leads to increased
storage costs to prevent exposure to the ambient atmosphere
11,12
. The
amount of residual lithium compounds increases with increases in the
Ni content of the cathode materials and production mass per batch;
therefore, regulating them is a crucial step in developing Ni-richer
layered cathode materials for the industrial field12–14.
A variety of different methods have been employed to eliminate
the residual lithium on cathode surfaces and can be mainly sorted into
two categories: coating through reactions with residual lithium and
Received: 9 June 2023
Accepted: 16 October 2023
Published online: 16 November 2023
Check for updates
1Department of Energy Engineering, Hanyang University, Seoul, South Korea. 2Department of Battery Engineering, Hanyang University, Seoul,
South Korea. e-mail: yksun@hanyang.ac.kr
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... For this reason, ex situ characterization techniques are usually implemented to probe the surface structures of the catalysts, e.g., scanning electron microscopy (SEM), X-ray diffraction (XRD), surface-sensitive Brunauer-Emmett-Teller (BET) techniques, X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and the extended X-ray absorption fine structure (EXAFS) technique. From the reported literature [15][16][17][18], one can see that the surface atomic structures, under either ex situ or in situ conditions, are far from being clarified in terms of understanding their catalytic performance. ...
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