The global battery market was valued at 120.4 billion USD in 2020 and is estimated to continuously grow to 279.7 billion USD by 2027 at a compound annual growth rate of 12.8%. (1) This growth is mostly attributed to the expansion of the front industries, such as the electric vehicle (EV) and energy storage system sectors, which require much larger batteries than the portable electronic device sector. Despite the worldwide downturn in car sales owing to the COVID-19 pandemic, 3 million EVs were sold in 2020 because of supportive government subsidies and a continuous fall in the price of Li-ion batteries (LIBs). (2) However, in a few years the supply of raw materials for batteries may not meet the demand. In particular, Co is the most problematic component of LIBs owing to its unstable supply chain as well as ethical issues surrounding its mining. Specifically, the majority of Co is produced as a byproduct of Ni and Cu mines, and more than half of the Co reserves are geographically localized in the Democratic Republic of Congo. (3−5) The consequent price volatility of Co requires reducing Co dependency and even developing Co-free chemistries for LIBs (Figure 1). Cobalt is considered an indispensable element in LIB cathode chemistry. Since the LiCoO₂ cathode was first incorporated into commercial LIBs in 1991, the use of Co has continued in the Li[Ni_1-x-yCo_xMn_y]O₂ (NCM) and Li[Ni_1-x-yCo_xAl_y]O₂ (NCA) cathode materials currently employed in LIBs. (6) Cobalt is known to improve the electronic and ionic conductivities and structural stability of layered cathodes, ensuring the high performance and durability of LIBs. In addition, Co leads to the formation of highly crystalline layered cathodes by suppressing the cationic intermixing of Li and Ni ions. Despite many early efforts to find alternatives, no element has fully replaced Co in layered cathodes, maintaining the LIB industry’s heavy reliance on Co. Recently, the Co content in cathode compositions has been gradually decreasing as Ni-rich NCA and NCM cathode materials have become mainstream. (6) Moreover, with the strong pressure to reduce the amount of Co in current LIBs, Ni-rich, Co-free Li[Ni_1−xMn_x]O₂ (NM) cathodes have attracted significant interest from both academia and industry. In recent studies, Co-free NM cathodes have shown considerable promise for practical applications, and their cycling stability has been impressive, particularly under harsh cycling conditions. (7) One reason for their stable cycling performance is that the increased presence of stable Mn⁴⁺ enhances their structural and chemical stability. Replacing Co with Mn renders cathode particles resistant to the microstrain generated during electrochemical reactions, thereby suppressing the evolution of severe microcracks. The long-term cycling performance of Ni-rich, Co-free NM cathodes has been demonstrated in full cells, suggesting their commercial viability for LIB cathodes. However, Co-free NM cathodes suffer from a poor rate capability, especially during low-temperature operation, which is an unsolved problem for commercialization. (8) Although their poor rate performance can be partially improved by increasing the Ni content, it is imperative to develop strategies that improve the ionic and electronic conductivities of Co-free NM cathodes for high rate performance. A new NM chemistry that substitutes other constituent elements could be a solution to the poor rate performance. Instead of NM, some researchers have investigated LiNiO₂ (LNO) as a Co-free cathode material, but its extremely high Ni content leads to poor cycling stability, which remains a major obstacle. (9) Researchers have attempted to address the rapid capacity fading of LNO cathodes by appropriately doping elements into LNO. However, the question remains whether applying the conventional approaches for modifying NCM and NCA cathodes will work equally well for Co-free cathode chemistries, especially in terms of the long-term cycling performance. Figure 1. Cobalt: the most expensive raw material in LIBs. As part of another class of cathode structures for LIBs, LiFePO₄ (LFP) with an olivine structure is widely used in industry for Co-free cathodes. (10) Its main advantage is that Fe costs far less than Ni and Co. In addition, owing to the strong P–O bonds, LFP cathodes exhibit remarkable structural and thermal stability. However, LFP is nearly an insulator, with an electronic conductivity of ∼10^–8 S cm^–1, and its energy density is inferior to those of Ni-rich NCA and NCM cathodes. Fortunately, its low conductivity can be increased by carbon coating and controlling the particle size. However, these methods must be carefully optimized; otherwise, they can decrease the volumetric energy density. To increase the energy density, olivine-type LiMn_xFe_1-xPO₄ (LMFP) derivatives could be used as possible substitutes for LFP. (11) Because Mn has a higher redox potential than Fe, the LMFP cathode exhibits two plateaus in the characteristic curves during the charge and discharge reactions. These olivine cathodes have different benefits in terms of price, cycle life, and thermal stability, but their main weakness is that their energy density is lower than those of Ni-rich layered cathodes. Meanwhile, spinel-type LiMn₂O₄ (LMO) is the other Co-free cathode material that has been commercialized. Because of its three-dimensional Li⁺ diffusion path, the LMO cathode performs well at high C-rates. However, LMO remains on the outer fringes of cathode production for LIBs owing to its problematic low energy density and poor cycling stability. Partially substituting Mn with Ni to synthesize LiNi_0.5Mn_1.5O₄ (LNMO) can improve the energy density of spinel cathodes by enabling a higher operating voltage of ∼5 V. (12) Although a higher energy density and good rate performance are attractive for EV batteries, the practical use of LNMO cathodes also requires an appropriate electrolyte for high-voltage operation and good cycling stability. Eliminating Co from the cathode chemistry would help stabilize the price of LIBs. The challenge is to realize a Co-free cathode chemistry without sacrificing the energy density, cycle life, or rate capability. Although Co-free cathodes do not currently surpass the already adopted NCM and NCA cathodes in several aspects, the demand for Co-free cathodes will increase to satisfy diverse commercial needs depending on the application. Therefore, a comprehensive understanding of their fundamental properties must take precedence in the development of Co-free cathode materials. Based on this understanding, appropriate strategies can be designed to overcome the shortcomings of Co-free cathodes. Besides the cathode itself, the research should be combined with a search for other battery components suitable for Co-free LIBs. Once the Co-free LIBs are successfully developed, these batteries and the industries that rely on them will become more environmentally sustainable and cost-effective. To this end, research from various perspectives is needed for the further development of Co-free batteries, and ACS Energy Letters welcomes your valuable experimental findings and deep theoretical insights. This article references 12 other publications. This article has not yet been cited by other publications. Figure 1. Cobalt: the most expensive raw material in LIBs. This article references 12 other publications.

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锂离子电池无钴化学品的兴起

2020 年全球电池市场价值 1204 亿美元，预计到 2027 年将持续增长至 2797 亿美元，年复合增长率为 12.8%。(1) 这一增长主要归因于前沿产业的扩张，例如电动汽车 (EV) 和储能系统领域，这些领域需要比便携式电子设备领域更大的电池。尽管由于 COVID-19 大流行导致全球汽车销量下滑，但由于政府的支持性补贴和锂离子电池 (LIB) 价格的持续下跌，2020 年售出了 300 万辆电动汽车。(2) 然而，几年后电池原材料的供应可能无法满足需求。特别是，由于其不稳定的供应链以及围绕其采矿的道德问题，Co 是 LIB 中最成问题的组成部分。具体来说，大部分钴是镍矿和铜矿的副产品，一半以上的钴储量在地理上位于刚果民主共和国。(3-5) 随之而来的钴价格波动需要减少对钴的依赖，甚至需要开发用于锂离子电池的无钴化学物质（图 1）。钴被认为是锂离子电池正极化学中不可或缺的元素。自从 LiCoO₂正极于 1991 年首次被纳入商业 LIB，Co 的使用继续在 Li[Ni _{1 - xy} Co _x Mn _y ]O ₂ (NCM) 和 Li[Ni _{1- xy} Co _x Al _y ]O ₂(NCA) 目前用于 LIB 的正极材料。(6) 钴可以提高层状正极的电子和离子电导率和结构稳定性，确保锂离子电池的高性能和耐用性。此外，Co 通过抑制 Li 和 Ni 离子的阳离子混合，导致形成高度结晶的层状正极。尽管早期努力寻找替代品，但层状正极中没有一种元素可以完全取代钴，这使得锂离子电池行业对钴的依赖程度仍然很高。最近，随着富镍 NCA 和 NCM 正极材料越​​来越多，正极成分中的 Co 含量逐渐下降。主流。(6) 此外，在降低现有锂离子电池中钴含量的强大压力下，富镍、无钴的 Li[Ni _{1− x} Mn _x ]O₂ (NM) 阴极引起了学术界和工业界的极大兴趣。在最近的研究中，无钴 NM 正极在实际应用中显示出相当大的前景，并且它们的循环稳定性令人印象深刻，特别是在恶劣的循环条件下。(7)其循环性能稳定的原因之一是稳定 Mn ^{4+的存在增加}增强了它们的结构和化学稳定性。用 Mn 代替 Co 使正极颗粒能够抵抗电化学反应过程中产生的微应变，从而抑制严重微裂纹的发展。富镍、无钴 NM 正极的长期循环性能已在全电池中得到证明，表明它们在 LIB 正极中具有商业可行性。然而，无钴 NM 正极的倍率性能较差，尤其是在低温运行期间，这是商业化尚未解决的问题。(8) 虽然可以通过增加 Ni 含量来部分改善其较差的倍率性能，但必须制定策略来提高无钴 NM 正极的离子和电子电导率以实现高倍率性能。替代其他组成元素的新 NM 化学可能是解决不良倍率性能的方法。一些研究人员没有研究 NM，而是研究了 LiNiO₂（LNO）作为无钴正极材料，但其极高的Ni含量导致循环稳定性差，这仍然是一个主要障碍。(9) 研究人员试图通过在 LNO 中适当掺杂元素来解决 LNO 正极的快速容量衰减问题。然而，问题仍然是应用传统方法来修饰 NCM 和 NCA 正极是否同样适用于无钴正极化学，特别是在长期循环性能方面。图 1. 钴：锂离子电池中最昂贵的原材料。作为另一类 LIB 阴极结构的一部分，LiFePO ₄具有橄榄石结构的（LFP）在工业中广泛用于无钴正极。(10) 其主要优点是 Fe 的成本远低于 Ni 和 Co。此外，由于 P-O 键强，LFP 正极表现出显着的结构和热稳定性。然而，LFP 几乎是一种绝缘体，其电子电导率约为 10 ^-8 S cm ^-1，其能量密度不如富镍 NCA 和 NCM 正极。幸运的是，它的低电导率可以通过碳涂层和控制粒径来增加。但是，必须仔细优化这些方法；否则，它们会降低体积能量密度。为了提高能量密度，橄榄石型 LiMn _x Fe _{1- x} PO ₄(LMFP) 衍生物可以用作 LFP 的可能替代品。(11) 由于 Mn 比 Fe 具有更高的氧化还原电位，LMFP 正极在充电和放电反应期间的特性曲线中表现出两个平台。这些橄榄石正极在价格、循环寿命和热稳定性方面具有不同的优势，但它们的主要弱点是它们的能量密度低于富镍层状正极。同时，尖晶石型LiMn ₂ O ₄ (LMO)是另一种已经商业化的无钴正极材料。由于其三维 Li ⁺扩散路径，LMO 阴极在高 C 率下表现良好。然而，由于 LMO 存在能量密度低和循环稳定性差等问题，LMO 仍处于 LIB 正极生产的边缘。Ni部分取代Mn合成LiNi _0.5 Mn _1.5 O ₄（LNMO）可以通过提高~5 V的工作电压来提高尖晶石正极的能量密度。（12）虽然更高的能量密度和良好的倍率性能对电动汽车电池很有吸引力，但LNMO正极的实际使用还需要适当的用于高压操作和良好循环稳定性的电解液。从阴极化学中去除钴将有助于稳定锂离子电池的价格。挑战在于在不牺牲能量密度、循环寿命或倍率能力的情况下实现无钴阴极化学。尽管目前无钴正极在几个方面都没有超过已经采用的 NCM 和 NCA 正极，但根据应用的不同，对无钴正极的需求将会增加，以满足不同的商业需求。所以，在开发无钴正极材料时，必须先全面了解它们的基本特性。基于这种理解，可以设计适当的策略来克服无钴正极的缺点。除了正极本身，该研究还应与寻找适合无钴锂离子电池的其他电池组件相结合。一旦成功开发出无钴锂离子电池，这些电池和依赖它们的行业将变得更具环境可持续性和成本效益。为此，无钴电池的进一步发展需要多方面的研究，该研究应与寻找适合无钴锂离子电池的其他电池组件相结合。一旦成功开发出无钴锂离子电池，这些电池和依赖它们的行业将变得更具环境可持续性和成本效益。为此，无钴电池的进一步发展需要多方面的研究，该研究应与寻找适合无钴锂离子电池的其他电池组件相结合。一旦成功开发出无钴锂离子电池，这些电池和依赖它们的行业将变得更具环境可持续性和成本效益。为此，无钴电池的进一步发展需要多方面的研究，ACS 能源快报欢迎您的宝贵实验发现和深刻的理论见解。本文引用了其他 12 种出版物。这篇文章尚未被其他出版物引用。图 1. 钴：锂离子电池中最昂贵的原材料。本文引用了其他 12 种出版物。