Degradation mechanism and performance enhancement strategies of LiNixCoyAl 1 −x−yO 2 ( x ≥ 0.8) cathodes for rechargeable lithium-ion batteries: a review

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A b s t r a c t

Nonetheless, the structural and interfacial instability leads to their poor cyclability and inferior thermal stability, which needs to be urgently addressed prior to their further practical applications.

それにもかかわらず、構造的および界面の不安定性は、それらの不十分なサイクル性および劣った熱安定性につながり、それらのさらなる実用化の前に緊急に対処する必要がある。

In this review, we give a brief introduction about the degradation mechanism of layered NCA cathodes during charge-discharge processes and summarize proposed performance enhancement strategies, which may contribute to provide important ideas to design and construct integrated layered NCA cathodes for high-energy density battery

このレビューでは、充放電プロセス中の層状NCAカソードの劣化メカニズムについて簡単に紹介し、提案された性能向上戦略を要約します。これは、高エネルギー密度電池用の統合層状NCAカソードを設計および構築するための重要なアイデアを提供するのに役立つ可能性があります。

However, the NCA cathodes simultaneously suffer from the unacceptable performance fading during cycles, which can be partly ascribed to the thermodynamic instability in the full state of charge and partly to the residual lithium compounds with strong alkalinity originated from the synthesis and storage processes →ref [54 – 57].

ただし、NCAカソードは、サイクル中の許容できない性能低下に同時に悩まされます。これは、完全な充電状態での熱力学的不安定性と、に起因する強いアルカリ性を持つ残留リチウム化合物に一部起因する可能性があります。 合成および保管プロセス

in which thesurface coating and ion doping modification effectively mitigate these problems
→ref [58 – 61].
the coating materials can clear up the residual lithium compounds on the surface of the cathodes as well as partially doping into the surface-near lattice to modify the structure of the cathode materials
→ref [65, 66]

The origins of the unacceptable performance deterioration of the NCA cathodes primarily include cationic mixing, phase transition, residual lithium compounds, and microcracks, etc.

NCAカソードの許容できない性能低下の原因には、主にカチオン混合、相転移、残留リチウム化合物、マイクロクラックなどがあります

1.Cationic mixing

Among transition metal ions, the Ni2+ ions easily occupy the positions of the Li + ions owing to the similar radius between the Li + (0.76 Å) and Ni 2+ (0.69 Å) ions.

遷移金属イオンの中で、* Li +（0.76Å）とNi2+（0.69Å）の間の半径が類似しているため、 Ni2 + イオンは Li + **イオンの位置を簡単に占めます。 *

the Ni/Li mixing could be ultimately attributed to the thermodynamic instability of the structure of the NCA materials, which induces the reduction of Ni3+ from high valence to Ni 2+ and leads to a net loss of Li and O in the form of Li2 O and O2 . Moreover, the cationic mixing is generally aggravated due to the increase of Ni 2+ ions with the increase of the discharge degree [72, 76–78].

Ni2+/Li2+混合会导致热力学的不稳定，导致价带的Ni3+退到Ni2+，导致Li和O元素的消耗因为会形成Li2O和O2.且这种混合随着放电时Ni2+的增加而恶化。（放电，是cathode流出电流那么cation+在电解液中进入cathode，即Li+，那么？？？）

2.Phase transition

Layered-spinel-rock salt phase transition is the inevitable result of the cationic mixing, which causes the loss of oxygen from the NCA cathodes [77, 79]. The oxygen intermediates on the surface are rapidly depleted by the electrolyte solvents, and the oxygen evolution processes inside the cathodes are more kinetically hindered. Therefore, the layered-spinel-rock salt phase transition is much more severe near the surface of particles and cracks than that in the bulk of the cathodes, accompanying with the chemical oxidations of electrolyte solvents and resultant gas generation [80, 81]

表面の酸素中間体は電解質溶媒によって急速に枯渇し、カソード内の酸素発生プロセスはより速度論的に妨げられます。 したがって、層状スピネル岩塩の相転移は、電解質溶媒の化学的酸化とその結果としてのガス発生を伴い、カソードの大部分よりも粒子と亀裂の表面近くではるかに深刻です[80、81]。

Neither NiO rock salt phase nor solvent decomposition products is electrochemically active and ionic conductive, which form a thick and highly resistive layer on the surface of the cathodes, thereby notoriously increasing the transfer impedance of the battery. Moreover, the layered-spinel-rock salt phase transition usually aggravates as the increases of operating voltage and cycle number, thereby resulting in declines of the capacity and working potential.

NiO岩塩相も溶媒分解生成物も電気化学的に活性でイオン伝導性ではなく、カソードの表面に厚くて高抵抗の層を形成し、それによってバッテリーの伝達インピーダンスを高めることで有名です。 さらに、層状スピネル岩塩の相転移は、通常、動作電圧とサイクル数の増加に伴って悪化し、その結果、容量と動作電位が低下します。

3.Residual lithium compounds

The residual lithium compounds appearing on the surface of the NCA materials are mainly derived from two aspects. On the one hand, excess amount of LiOH is usually introduced into cathode materials during the preparation process for offsetting the loss of Li2O by sublimation and restraining the cationic mixing ---ref[74].

NCA材料の表面に現れる残留リチウム化合物は、主に2つの側面に由来します。 一方では、通常、昇華によるLi2 Oの損失を相殺し、カチオン混合を抑制するために、準備プロセス中に過剰量のLiOHがカソード材料に導入されます。
过量的LiOH来源于烧结过程中为了防止生化消耗LiO2而引入，那么过量的Li就会在粒子表面。

On the other hand, intermediate LiNiO 2 in cathode materials can easily react with H2 O and CO 2 from the air to yield LiOH and Li2 CO 3 by-products as Eqs. (4–5) during storage times。

一方、カソード材料の中間LiNiO2は、空気中のH2OおよびCO2と容易に反応して、保管時間中にLiOHおよびLi2CO3副生成物を生成する可能性があります。

Aforementioned residual lithium compounds usually lead to some problems such as slurry gelation during the cathode materials coating and battery swelling processes. LiOH can easily react with the dehydrofluorination of polyvinylidene fluoride binder to form (CH=CF) n polymer and H2 O according to Eq. (6), in which the C=C unsaturated bonds tend to crosslink polymerization and trigger the slurry gelation .

前述の残留リチウム化合物は、通常、カソード材料のコーティング中のスラリーのゲル化や電池の膨潤プロセスなどの問題を引き起こします。 LiOHは、ポリフッ化ビニリデンバインダーの脱フッ化水素と容易に反応して、（CH=CF）nポリマーとH2Oを形成します。このポリマーでは、C = C不飽和結合が重合を架橋し、スラリーのゲル化を引き起こす傾向があります。

In addition, Li2 CO 3 is electrochemically oxidized to O 2 and CO 2 under a high potential (> 4.3 V vs. Li+ /Li) according , which can induce the battery swelling .

さらに、Li2 CO 3は電気化学的に酸化されて高電位（> 4.3V対Li + / Li）でO2とCO2になり、バッテリーの膨張を引き起こす可能性があります。