Imagining the modern world and everyday human functioning without electrochemical power sources is difficult. The rapid development of electronics, and the growing market of modern mobile devices, largely depend on the availability of cheap, efficient and safe power sources with high energy density. A strong trend in electromobility development and the growing demand for electric propulsion systems has also been observed in recent years. Moreover, the progressive climate change, more restrictive legal requirements in the field of environmental protection, rising prices and decreasing fossil fuel resources are the driving force of the research on new energy storage and conversion systems. Nowadays, lithium-ion batteries (LIBs) are the most promising. Since their introduction by Sony in 1991, lithium-ion batteries have been with us at almost every step. Beneficial Li-ion battery characteristics, such as high energy density, high operating voltage, low self-discharge rate, good cyclic stability and a wide operating temperature range, are reasons for the constant interest of scientists and industry in the further development of this great invention awarded with the Nobel Prize in 2019 (Nitta et al., 2015; Zubi et al., 2018; Armand et al., 2020). One of the biggest challenges is finding new solutions to reduce costs and improve the safety of use and operating parameters (capacity, operating voltage, stability, and reversibility of the charging/discharging process). However, the possibility of achieving higher cell power and capacity lies mainly in improving the cathode material, being one the most essential part of the battery constituting over 40% of its total cost. Therefore, work is still ongoing on improving the existing cathode materials and developing new materials that could fulfil this role. An efficient cathode compound must exhibit many specific properties, i.e., the presence of ions easily undergoing redox reactions, a high redox potential vs lithium, capability to a high speed and reversible lithium intercalation/deintercalation process, a high electronic and ionic conductivity, chemical stability under operating conditions, no reactions with the electrolyte, low cost, simplicity of the synthesis process, low toxicity. Lithium transition metal oxides of the layered structure reveal significant potential in this respect.

The most prospective cathode materials are those with high nickel content (Xia et al., 2018; Li et al., 2020), which is their great advantage by providing high capacity. Nevertheless, it has also become a source of considerable issues causing significant chemical, structural, and thermal instability. The deterioration of the electrochemical performance is mainly related to such phenomena as Ni/Li cation mixing (structural disordering), residual lithium compounds, irreversible phase transitions, microcracking of secondary particles or transition metal ions dissolution (Zhang, 2020). Poor stability and structure ordering are related to the tendency of low-valence Ni²⁺ (0.69 Å) ions to migrate from 3b sites to 3a sites (interlayer spaces) usually occupied by Li⁺ (0.74 Å) ions of similar size. As a result, a severe capacity decrease, hindered intercalation/deintercalation of Li ions, difficulties with obtaining proper cathode compositions and changes of crystallographic structure from layered by spinel into a NiO-like rock-salt phase may be observed (Zheng et al., 2019). Nonetheless, the significant benefits of using Ni-rich cathode materials constantly propel scientists to improve their properties (Xie et al., 2023). The greatest challenge is optimizing their production procedure in terms of increasing the Li⁺ diffusion coefficient and counteracting Ni/Li cation mixing. Overcoming these significant limitations of Ni-rich cathode materials is possible by morphological or structural modifications based on elemental substitution (doping) and surface coating (Guan et al., 2020; Tan et al., 2020; Yan et al., 2020; Cui et al., 2023).

In this paper, the properties of Ni-rich cathode materials modified by rare earth elements (REEs) are briefly reviewed. The work focuses exclusively on the rare earth elements as a highly prospective group of modifiers with the desired characteristics, which have been sparingly described in the literature so far. First, the most common methods of modifications of layered cathode materials were introduced in general. Then, the recent progress in the application of rare earth elements in Ni-rich cathode materials modification was described. The influence of a given element on electrochemical performance was summarized. Finally, new strategies for cathode materials improvement have been indicated.

The most common and effective method to improve the electrochemical properties of Ni-rich cathode materials is doping. It is based on the combination of the specific features of a host material and added foreign atom modifying a broad spectrum of cathode materials properties, e.g., capacity, electrical conductivity, capacity retention or high-temperature/high-voltage performance. According to the model described in the literature (Purwanto et al., 2018), a doping process may be applied in two ways: 1) indirectly–during the preparation of a cathode precursor or 2) directly—by adding the selected dopant to the prepared cathode powder precursor. In the first case, doping cations may enter Ni sites. They should be ions characterized by high or stable valence or ions with a reduction potential beyond the cathode’s operating potential range. These dopants introduced into the oxide layers are usually electrochemically inactive, have large ionic radii and form strong bonds with oxygen, stabilizing the cathode material structure. For Ni-rich materials, the following cations are usually used: Al³⁺, B³⁺, La³⁺, Eu³⁺, Nd³⁺, Sn⁴⁺, Ce⁴⁺, Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, V⁵⁺, Mo⁶⁺ (Cui et al., 2023). In direct doping, cations preferentially occupy sites generally occupied by Li⁺ ions. Usually, these are cations with lower valences (e.g., Na⁺, K⁺, Mg²⁺) acting as pillars stabilizing materials structure, i.e., holding the layered structure and preventing local collapse caused by Ni/Li cation mixing (He et al., 2019). Replacing some lithium ions with those of larger radius should also promote an expansion of Li⁺ pathways by enlarging layer spacing and improving the lithium diffusion kinetics during the cell operation (Xie et al., 2016).

The prominent properties enhancement may also occur upon surface coating (Guan et al., 2020). Thin surface layers, intentionally produced or formed on the electrode material surface during its modification by doping, can stabilize the physical structure of materials and the interface between the active cathode material and the electrolyte. They protect the electrode surface against harmful side reactions with the electrolyte and oxygen release. The passivation layer of high ionic conductivity may also enhance material conductivity, prevent electrode corrosion by electrolyte and improve electrode stability. Conductive carbon materials, metal oxides and metal phosphates coatings are commonly used (Ahaliabadeh et al., 2022).

Surprisingly, rare earth elements (REEs) are the group of modifiers that are the least applied and described in the literature so far, even though they possess all the necessary features qualifying them as an effective dopant of layered cathode materials, i.e., 1) large ionic radius, 2) a tendency to form strong bonds with oxygen (high affinity to oxygen), 3) electrochemical inactivity (no participation in oxidation/reduction reactions during cycling). Rare earth elements are certainly of interest both from a scientific perspective and for practical applications but have not received as much attention as transition metal dopants. The reason for this may be the high prices of REEs because the isolation and large-scale production of individual rare earth compounds remain challenging (Zheng et al., 2022). Difficulties with their full incorporation into the host lattice are another challenge. The chemical similarity between the host cations and the rare earth dopants determines the effectiveness of doping. The size of ions is thus one of the most critical parameters to be considered in substituting host cations with rare earth dopants. In general, ions of similar size can substitute for each other in a crystalline compound without any noticeable change in the crystal structure (Zheng et al., 2022). Rare earth ions are much larger than transition metal ions which form layered Ni-rich structures. Hence, in some cases, the doping into the cathode lattice is not successful, and the excessive REE ions form impurities that can adversely affect the cathode properties (Ahaliabadeh et al., 2022). However, the uniqueness of rare earth dopants is that even in a small amount, they can affect a cathode material differently and change specific properties, e.g., increase capacity, enhance cyclic performance, improve rate capability, reduce the cation mixing effect, and protect against structural changes or adverse interaction with the electrolyte. Herein, the selected up-to-date publications regarding the effects of modification by selected rare earth elements (Y, Ce, La, Nd, Eu) on Ni-rich cathode materials performance were introduced and summarized in Supplementary Table S1.

The recent achievements in the modification of Ni-rich layered cathode materials using rare earth elements were briefly overviewed. Rare-earth elements-modified cathodes have offered promising countermeasures against many degradation mechanisms observed for Ni-rich materials. However, the potential of these elements in the layered cathode materials design has not yet been fully appreciated and exploited. Suited chemistry and application of appropriate modification methods are the keys to significant improvement and extraction of the full potential of Ni-rich cathode materials. The most common strategies, i.e., doping or coating, usually affects only selected material properties and only to a limited extent. Hence, the implementation of combined methods, e.g., simultaneous doping and coating, dual-doping (two different modifiers substituted in one position in the structure), dual-site doping (one modifier substituted in two different positions in the structure), multicomponent doping or multi-element surface coating causing strong synergistic effects, seems to be the most reasonable and promising strategies. The design of high-performance cathode materials exhibiting the best possible working parameters regarding capacity, stability and potential is of paramount importance in the face of today’s challenges connected with the growing energy demand.