Quantifying Electrochemical Degradation in Single-Crystalline <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mrow><mml:mi>Li</mml:mi><mml:mi>Ni</mml:mi></mml:mrow><mml:mrow><mml:mn>0.8</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Mn</mml:mi><mml:mrow><mml:mn>0.1</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Co</mml:mi><mml:mrow><mml:mn>0.1</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:math>–Graphite Pouch Cells through <i>Operando</i> X-Ray and Postmortem Investigations

Abstract

Layered nickel-rich lithium transition-metal oxides (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:msub><a:mrow><a:mi>Li</a:mi><a:mi>Ni</a:mi></a:mrow><a:mi>x</a:mi></a:msub><a:msub><a:mi>Mn</a:mi><a:mi>y</a:mi></a:msub><a:msub><a:mi>Co</a:mi><a:mrow><a:mn>1</a:mn><a:mo>−</a:mo><a:mi>x</a:mi><a:mo>−</a:mo><a:mi>y</a:mi></a:mrow></a:msub><a:msub><a:mrow><a:mrow><a:mi mathvariant="normal">O</a:mi></a:mrow></a:mrow><a:mn>2</a:mn></a:msub></a:math>; where ≥ 0.8), with single-crystalline morphology, are promising future high-energy-density <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><e:mi>Li</e:mi></e:math>-ion battery cathodes due to their ability to mitigate particle-cracking-induced degradation. This is due to the absence of grain boundaries in these materials, which prevents the build-up of bulk crystallographic strain during electrochemical cycling. Compared to their polycrystalline counterparts, there is a need to study single-crystalline <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><h:mi>Ni</h:mi></h:math>-rich cathodes using x-ray methods in uncompromised machine-manufactured industrylike full cells to understand their bulk degradation mechanisms as a function of different electrochemical cycling protocols. This can help us identify factors to improve their long-term performance. Here, through in-house x-ray studies of pilot-line-built <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><k:msub><k:mrow><k:mi>Li</k:mi><k:mi>Ni</k:mi></k:mrow><k:mrow><k:mn>0.8</k:mn></k:mrow></k:msub><k:msub><k:mi>Mn</k:mi><k:mrow><k:mn>0.1</k:mn></k:mrow></k:msub><k:msub><k:mi>Co</k:mi><k:mrow><k:mn>0.1</k:mn></k:mrow></k:msub><k:msub><k:mrow><k:mrow><k:mi mathvariant="normal">O</k:mi></k:mrow></k:mrow><k:mn>2</k:mn></k:msub></k:math>–graphite A7 pouch cells, it is shown that their electrochemical-capacity fade under harsh conditions (2.5–4.4 V and 40 °C for 100 cycles at a C/3 rate) primarily stems from the high-voltage reconstruction of the cathode surface from a layered to a cubic (rock-salt) phase that impedes the <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><o:msup><o:mi>Li</o:mi><o:mo>+</o:mo></o:msup></o:math> kinetics and increases cell impedance. Postmortem electron and x-ray microscopy show that these cathodes can withstand severe anisotropic structural changes and show no cracking when cycled under such conditions. Comparing these results to those from commercial <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><r:mi>Li</r:mi></r:math>-ion cells with surface-modified single-crystalline <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><u:mi>Ni</u:mi></u:math>-rich cathodes, it is identified that cathode surface passivation can mitigate this type of degradation and prolong cycle life. In addition to furthering our understanding of degradation in single-crystalline <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><x:mi>Ni</x:mi></x:math>-rich cathodes, this work also accentuates the need for practically relevant and reproducible fundamental investigations of <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><ab:mi>Li</ab:mi></ab:math>-ion cells and presents a methodology for achieving this. Published by the American Physical Society 2024

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