Multicomponent or high-entropy alloys (HEA) differ from traditional alloys in that they consist of multiple principal elements. HEAs were introduced in 2004 independently by authors Cantor and Yeh (Cantor et al., 2004; Yeh et al., 2004). Since then, high-entropy alloys have primarily been studied in the context of metallurgy, where they are known to provide desirable properties such as high strength and corrosion resistance (Pickering and Jones, 2016; Zhang, 2019). The role entropy plays in the phase stability of these compounds can be understood in terms of the Gibbs free energy of mixing (ΔG_mix) (Rost et al., 2015; Widom, 2018; Akrami et al., 2021; Xiang H. et al., 2021; Chen et al., 2021; Fu et al., 2021; Ma et al., 2021): Δ G m i x = Δ H m i x − T Δ S m i x (1) where ΔH_mix is the mixing enthalpy, ΔS_mix is the mixing entropy, and T is the absolute temperature. If the TΔS_mix term dominates the enthalpy term (ΔH_mix), the overall ΔG_mix becomes negative and entropy stabilization has been established. The mixing entropy (ΔS_mix) includes many entropic terms, however the configurational entropy (ΔS_conf) is usually the dominant contribution (Chen et al., 2021). Configurational entropy for an ideal solid solution is proportional to the number of components n: (Chen et al., 2021; Ma et al., 2021) Δ S c o n f = − R ∑ i = 1 n x i l n x i (2) where R is the ideal gas constant and x _i is the component mole fraction. When all components are present in equimolar proportion, the configurational entropy reaches its maximum and Eq. (2) simplifies to the equation below: (Chen et al., 2021; Ma et al., 2021) Δ S c o n f = R l n ( n ) (3)

There are two formal definitions of high-entropy alloys. First, the compositional based definition states that the alloy must contain at least five elements, with each having an atomic percentage between 5% and 35% (Zhang, 2019; Ma et al., 2021). This definition has often been used in physical metallurgy where it is common to dope alloys with small quantities of elements. Second, the entropy-based definition states that an alloy is high-entropy when ΔS_conf is greater than 1.5R (Zhang, 2019; Ma et al., 2021). This criterion is satisfied for a solid solution with five equimolar components, where the ΔS_conf of Eq. (3) simplifies to 1.61R.

Four “core effects” are believed to be responsible for the unique properties of high-entropy materials: the aforementioned entropy stabilizing effect, lattice distortion, sluggish diffusion, and the so-called cocktail effect (Yeh, 2015; Pickering and Jones, 2016). Lattice distortion results from the different atomic sizes of the constituent elements. This can provide benefits like increased alloy strength or allow for the storage of hydrogen into interstitial sites (Pickering and Jones, 2016; Ma et al., 2021). Sluggish diffusion refers to the low diffusion and phase transformation rates from the fluctuations of lattice potential energy (Yeh, 2015). Finally, the “cocktail effect” is the synergistic effect believed to result from the interplay between composition, structure, and microstructure (Yeh, 2015; Pickering and Jones, 2016). However, these “core effects” are not necessarily unique to high-entropy materials (HEM), and some authors have disputed their contribution to material properties (Pickering and Jones, 2016). HEMs have an additional economic benefit since they are often composed of inexpensive metals, which reduces reliance on any single element (e.g., Co.).

High-entropy oxides (HEO) are a newer class of HE material with numerous applications in energy storage. Like HEAs, entropy-based definitions have been proposed for HEOs, where the configurational entropy from both cations and anions is considered (Sarkar et al., 2018a; Sarkar et al., 2020b). Typically, the O^2- anion entropy contribution is zero, but the presence of oxygen vacancies or other anions (e.g., F⁻) can contribute to the system’s entropy (Sarkar et al., 2020b). In practice, it is difficult to determine whether a multicomponent oxide is truly entropy stabilized. The term “entropy-stabilized oxides” should be reserved for materials with a proven entropy-driven reversible phase transformation (Sarkar et al., 2020b). As a result, the term “high-entropy” is more often employed to describe an oxide system containing five or more equimolar cations.

In 2015, Rost et al. showed that the (NiCuZnMgCo)O HEO could be stabilized into a single-phase rock-salt structure (Rost et al., 2015). This was of interest to battery researchers as it allowed for the possibility of high-entropy electrodes, which are often oxide ceramics. The authors were able to show a reversible transformation from multi-phase to single-phase by varying calcination temperature. The transition was an endothermic process (ΔH>0); hence the system was entropy stabilized because the TΔS term must be large enough to overcome the positive enthalpy and render the Gibbs free energy negative. The removal of any one of the five components destabilized the solid solution, as seen in the X-ray diffraction (XRD) data of Figure 1A. This highlights the importance of five cations in entropy stabilized oxides. In addition, phase diagrams in Figure 1B confirm that the minimum transition temperature required for the formation of a single-phase occurs at maximum configurational entropy (Rost et al., 2015).

Today, the high-entropy concept has been applied to alloys, oxides, carbides, nitrides, borides, hydrides, sulfides, among others (Fu et al., 2021). Over 50 high-entropy oxides are known and take on a variety of crystal structures including rock-salt, perovskite, fluorite, spinel, and layered (Figure 1C) (Sarkar et al., 2020b; Akrami et al., 2021). As seen in Figure 1D, publications on the topic have accelerated over the past few years, including a number of reviews (Widom, 2018; Vaidya, Muralikrishna and Murty, 2019; Sarkar et al., 2020b; Akrami et al., 2021; Amiri and Shahbazian-Yassar, 2021; Chen et al., 2021; Fu et al., 2021; Lin et al., 2021; Ma et al., 2021; Wang Q. et al., 2021). For energy storage, HEOs have shown promising results in the fields of batteries, supercapacitors, electrochemical catalysis, hydrogen storage, hydrogen evolution reaction, oxygen evolution reaction, carbon dioxide conversion, and oxidation reactions involving methanol, ethanol, or formic acid (Sarkar et al., 2017b; Sarkar, et al., 2018a; Wang, et al., 2019a; Sarkar et al., 2019; Fu et al., 2021; Ma et al., 2021; Sturman et al., 2021; Sun et al., 2022). In battery research, HEMs are often used as electrode materials for Li-ion batteries, but they have also been used in solid electrolytes, Li-Sulfur and Na-ion batteries, as well as MXenes (Bérardan et al., 2016; Zhao et al., 2020; Akrami et al., 2021; Fu et al., 2021; Nemani et al., 2021; Xu et al., 2021; Yang et al., 2021; Etman, Zhou and Rosen, 2022). Consult Table 1 for a list of recent publications on high-entropy materials for battery applications. In this review, we have summarized the recent research on HEMs with an emphasis on their use as anodes and cathodes for Li-ion batteries. In addition, we discuss common synthesis methods, characterization techniques, electrochemical performance, and proposed lithiation and failure mechanisms.

The inclusion of fluorine into high-entropy rock-salts has been shown to increase their working potential to 3.4 V (Wang, et al., 2019b). This should be contrasted with a HEO baseline without fluorine, which has a working potential around 1.0 V. In addition, their preparation is simple and can be done by ball-milling a HEO precursor with LiF. XRD and Nuclear Magnetic Resonance (NMR) confirmed the successful incorporation of Li and F into the structure. These high-entropy rock-salt oxyfluorides have been used as cathode materials for lithium-ion batteries. Wang et al. reported an initial charge capacity of 161 mAh/g at C/10 for (Li_x (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)OF_x (Wang, et al., 2019b). This capacity was higher than a non-entropy stabilized LiNiOF_x baseline, despite containing a greater fraction of redox active nickel. However, the material suffered from a large first cycle irreversible capacity and an overall low CE (Wang, et al., 2019b). The authors attribute this capacity loss to electrolyte degradation, SEI formation, and side reactions. The oxyfluoride had an insertion-type lithiation mechanism, in contrast to the conversion-type mechanism seen in simple HEOs. Mg and Zn were redox inactive and remain divalent as shown with XPS. This cathode also exhibited a capacity increase during cycling.

The same group later studied various layered HEOs including LiNa(NiCoMnAlFe)₁O₂ (J. Wang, et al., 2020b)_. Figure 3A shows the material had an oxidation peak at 4.0 V, which is higher than the 3.8 V typically seen for standard Li(NiCoMn)₁O₂. However, the electrodes were reported to have a low capacity and a rapid capacity fade (<70 mAh/g after 20 cycles). The incorporation of a small amount of Na ions (Figure 3B) was done to improve the battery cycling by widening the diffusion channels. Inclusion of as little as 1 mol% Na (but not K or Rb) has also been reported to marginally improve capacity retention in conventional layered cathodes (He et al., 2019). Recently, Sturman et al. reported similar electrochemical performance for the Li(NiCoMnTiFe)₁O₂ layered HEO and observed overlithiation properties that were shown to reduce capacity decay (Sturman et al., 2021). Interestingly, a cathode with the same stoichiometry has also been reported for Na-ion batteries. The cathode was investigated both computationally and experimentally, where it was reported to have a stable capacity of 180 mA h/g (Walczak et al., 2022). Another recent HE layered cathode is described in the work of Wang et al. They used pulsed laser deposition to prepare epitaxial thin films of the layered LiCr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6O₂ (K. Wang et al., 2022a). The material was used as a cathode but had low capacity (<60 mAh/g) and high interfacial resistance. Zhen-Yi Gu et al. made a high-entropy derivative of the NASICON-type fluorophosphate cathode that was shown to increase operating voltage and electrochemical capacity in Na-ion batteries (Gu et al., 2022).

Ceder’s group used density functional theory (DFT) to estimate the mixing temperature of different transition metal combinations in cation-disordered rock-salts (DRX) (Lun et al., 2021). As a design strategy, they suggest the incorporation of 30% excess Li and the inclusion of fluorine to increase the redox reservoir. In addition, short-range order (SRO) was suppressed as the number of cations in the DRX increased. SRO was also reported to decrease with increasing synthesis temperature. Their TM6 cathode (i.e. Li_1.3Mn²⁺ _0.1Co²⁺ _0.1Mn³⁺ _0.1Cr³⁺ _0.1Ti_0.1Nb_0.2O_1.7F_0.3) had a high initial capacity of 307 mAh/g; however, capacity data was only reported for 20 cycles.

Lithium containing HEO rock-salts have displayed properties that are promising for solid electrolyte applications. Some have been reported to have a Li-ion conductivity several orders of magnitude greater than LiPON (Bérardan et al., 2016). Bérardan et al. studied the room-temperature ionic conductivity of rock-salt (MgCoNiCuZn)_1−x−yGa_yA_xO (with A = Li, Na, K) and reported a value exceeding 10⁻³ S cm⁻¹ (Bérardan et al., 2016). Later, the team studied (MgCoNiCuZn)_1−xLi_xO with varying concentrations of Li. They identified a charge compensation mechanism that begins with the oxidation of Co²⁺ to Co³⁺, followed by the formation of oxygen vacancies when the Li fraction exceeds x = 0.21 (Osenciat et al., 2019). High-entropy electrolytes have also been applied to Na-ion batteries, such as HE NASICON phosphate electrolytes (Wu et al., 2022).

Recent work on solid electrolytes includes HE garnets, argyrodites, and perovskites. Stockham et al. reported the successful synthesis of Ga-containing lithium garnet electrolytes composed of 9 and 11 elements (e.g., Ga_0.2Li_5.75La_2.5Nd_0.5Nb_0.65Ce_0.1Zr₁Ti_0.25O₁₂) (Stockham, Dong and Slater, 2022). The electrolytes were simple to prepare and had both good room temperature conductivity (0.2 mS cm⁻¹) and stability. As we have seen, most current research targets a high configurational entropy for the cations. However, recent work by Strauss et al. reports HE-polyanionic argyrodite-type solid electrolytes (Strauss et al., 2022). Unlike most publications, the authors reported a nonuniform distribution of elements. The high entropy was reported to lower activation energy for conduction but did not significantly affect ionic conductivity. Equimolar high-entropy perovskite electrolytes with varying Al content were reported but had a conductivity on the order of 10⁻⁷ S cm⁻¹ (Yazhou and Zhiren, 2022). The authors also investigated the material as an anode, however the use of heavy elements (i.e. La and Ba) resulted in a low capacity of 58 mAh/g. Other studies have also used perovskites as electrode materials. Yan et al. reported results for the less-studied high-entropy perovskite titanate (Yan et al., 2020). It had an initial capacity of 125 mAh/g followed by a large irreversible capacity, but a stable capacity of 80 mA h/g. The perovskite also had a gradual increase in capacity over 300 cycles, which the authors attribute to the gradual penetration of the electrolyte into the electrode and the formation of a gel-like film.

Confirmation of the homogeneous elemental distribution in HEOs has been an important part of their characterization since Rost et al. It has traditionally been supported with EDX or XRD analysis on the bulk powder. However, this may not be sufficient to detect traces of secondary phases or localized heterogeneities, which can affect the material’s property and electrochemical performance. Chellali et al. used Atomic Probe Tomography (APT) to study the atomic scale homogeneity of various transition metal, rare-earth metal, and perovskite-based HEOs prepared by a high-pressure compact method (Chellali et al., 2019). Their results confirm the random and homogeneous distribution of cations at the atomic level.

Moździerz et al. studied lattice distortion effects in (CoCuMgNiZn)_1-xLi_xO by increasing lithium levels during synthesis (x = 0–0.3) (Moździerz et al., 2021). The internal strain increased up to a Li composition of 0.2, after which the introduction of more Li released the internal strain through the formation of oxygen vacancies. The compound displayed mixed ionic-electronic conductivity upon the insertion of Li. Different mechanisms were proposed: intercalation-based insertion at high voltage and conversion or alloying at low voltage. Cheng et al. studied the effects of pressure on lattice distortion in (Ce_0.2La_0.2Pr_0.2Sm_0.2Y_0.2)O_2−δ HEO (Cheng et al., 2019). They observed pressure-induced transformations from the crystal state to the amorphous state below the melting point or glass-transition temperature. This property may allow for the development of ceramic-glass composites with applications in solid-state electrolytes.

Since 2018, authors have reported details regarding the partial conversion reaction in rock-salt HEOs. Sarkar suggested that certain cations (Co²⁺ and Cu²⁺) are involved in the conversion, whereas other cations (Mg²⁺) stabilize the structure (Sarkar, et al., 2018a). Operando XRD in Figure 4A shows the typical behaviour of a HE-rock-salt upon lithiation. Reflections from the rock-salt structure disappeared during the first lithiation due to the “formation of small crystallites” (which is often seen in conversion materials) (Sarkar, et al., 2018a). However, Selected Area Electron Diffraction (SAED) was still able to detect trace reflections from the rock-salt structure, providing evidence of a partially preserved structure that enables reversible lithium storage (Sarkar, et al., 2018a). Similar observations have been made in subsequent studies. Tavani et al. studied the lithiation mechanism of HEO (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O with operando X-ray Absorption Spectroscopy (XAS) to quantify the reduction of transition metal species (Tavani et al., 2020). For this material, upon lithiation, Cu²⁺ is first reduced to a mixture of 62% metallic Cu and 32% Cu₂O. Then, 75% of the Ni²⁺ is reduced to metallic Ni. Finally, around 62% of the Co²⁺ is reduced to metallic Co. The order of this sequence was noted to follow the redox potential of each element. These observations are also consistent with the work of Ghigna et al., where a lithiation/delithiation mechanism for (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O is proposed (Ghigna et al., 2020). The mechanism is illustrated in Figure 5.

They suggest a two-stage process beginning with the conversion of cations (Cu²⁺, Co²⁺, Ni²⁺), followed by the conversion and eventual alloying/de-alloying of Mg²⁺ and Zn²⁺. Surprisingly, it was found that ZnO is also involved in the conversion mechanism, as shown by the presence of metallic Zn. The reaction is incomplete and irreversible and ultimately leads to a collapse of the rock-salt structure, although 60% of the capacity remains. Wang et al. studied different quantities of Mg in the structure of (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O and found homogeneous single-phase compounds for only the Mg-containing oxides (Wang S.-Y. et al., 2021). However, the Mg-free sample still had good electrochemical performance. Operando X-Ray Microscope Spectroscopy revealed greater amounts of Mg were associated with a reduced pulverization.

More detailed characterization regarding HEO degradation is found in the work of Schweidler and Breitung. Schweidler et al. studied the lithiation of the (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)O anode with Acoustic Emission (AE) monitoring (Figure 4B) (Schweidler et al., 2021). The first cycle showed strong AE activity at low voltage, which was attributed to SEI formation. Later, they observed AEs mostly during the delithiation cycles, which is often where there are mechanical stresses associated with alloying/dealloying. Schweidler et al. also studied high-entropy oxyfluorides with AE (Schweidler et al., 2022). Acoustic activity was most prominent during the first cycle above 4.6 V and was attributed to mechanical degradation (crack propagation) and gas evolution. Further insight into the lithiation behaviour and side reactions that occur during the first cycle can be revealed by the gases released during cycling. For instance, oxygen release during cycling can be due to a phase transition in cathode materials. Breitung et al. investigated the gassing behaviour of the Li(Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)OF cathode and the (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)O anode with the help of pressure measurements and Differential Electrochemical Mass Spectrometry (DEMS) (Breitung et al., 2020). The anode evolved C₂H₄ gas mostly during the first cycle, which was attributed to the formation of a stable SEI. This was observed without the need for additives in the electrolyte, which are typically used in other high-capacity anodes like silicon. For the cathode, gassing behaviour was dependent on cut-off voltage. Onset of gas evolution was seen as early as 4.55 V for CO₂, however the 5 V cutoff was associated with the greatest gas evolution involving CO₂, CO, H₂, O₂, and POF₃. CO₂ and CO were attributed to oxidation of the electrolyte, which yields decomposition products that later form H₂. The LiPF₆ salt decomposition produced POF₃, and O₂ evolution was associated with surface lattice oxygen release. These observations are consistent with a mechanism similar to Li-rich disordered rock-salts. (Breitung et al., 2020).

Spinels are known to convert to rock-salt during synthesis upon the addition of lithium, as seen in Figure 4C. Wang et al. investigated the spinel to rock-salt transformation that occurs upon the incorporation of Li into HEOs during synthesis (J. Wang, et al., 2020c). The addition of lithium was accompanied with the oxidation of transition metals until oxidation was no longer possible, resulting in the transformation to rock-salt. The phase conversion in spinel (CrMnFeCoNi)₃O₄ during electrochemical cycling has also been studied at the atomic scale (Huang et al., 2021). During lithiation, the authors observed the formation of Cr_xFe_3-xO₄ and LiNi_xCo_1-xO₂ phases along with metallic Mn nanocrystals at 0.5 V. Further lithiation to 0.01 V led to the emergence of metallic Fe, Cr, Co., and Ni. Upon delithiation, the remaining spinel grains grew by incorporating nearby metal nanoparticles.

Several publications have studied the magnetic properties of HEOs. Witte et al. studied the magnetic properties of perovskite-type HEOs with rare-earth metals and observed mostly antiferromagnetic behaviour (Witte et al., 2019, 2020). More recently, Sarkar et al. reviewed the magnetic properties of HEOs (Sarkar, Kruk and Hahn, 2021). Sarkar also studied the electronic band structure of rare-earth metal based fluorite HEOs (Sarkar et al., 2020a). They highlighted the importance of synthesis temperature and atmosphere (reducing/oxidizing gas) on the band structure and number of oxygen vacancies.

The field of electrochemical energy storage has seen a recent growth in high-entropy materials. HEOs have the potential to be used as anode materials in Li-ion batteries as they offer very high capacity compared to the conventional graphite anode limited to 372 mAh/g. However, it must be stressed that these oxides typically have a large first cycle irreversible capacity and an electrochemical stability that is inferior to traditional anodes. In addition, they tend to operate at a higher potential (1 V—2 V) than graphite (0.2 V—0.3 V). For most applications, this is too high as it significantly drops the output voltage of the battery when coupled with conventional NMC layered cathodes (at around 4 V). Also, non-faradaic and surface pseudocapacitive contributions to the total capacity are still not fully understood. The presence of multiple cations, many of which have good catalytic activity, may react with the organic solvents and salts of the electrolytes. Alternatively, these novel electrode-electrolyte interactions may prove beneficial, as they may enable the formation of a highly stable SEI.

Disorder in cathode materials is known in the battery field to be a largely detrimental property. A classic example is LiNiO₂, where significant Li-Ni cation mixing causes degradation and eventual phase transition. The addition of other metal cations (i.e., Mn and Co.) has proven successful, as in the case of commercial NMC111. Here, stability was improved by these metal cations, and more careful cation selection may improve structural stability even further. However, it is not yet clear whether entropy stabilized cathodes are any less susceptible to degradation. In addition, certain promising high-capacity cathodes like DRXs require processing under argon which may limit their commercial viability. Nevertheless, HEO cathodes have the potential to stabilize the structure and allow metal cations to have greater stability and site energies that accommodate lithium. The use of five metal cations, without or with fluorination (such as in the case of DRX), or layered structures along with vacancies could lead to stable fully de-lithiated cathodes and hence higher capacities. Compared to HE-anodes, there is little work on HE-cathodes. Further work is needed to explore the full potential of these novel electrodes.

Finally, HEAs offer new opportunities to solve the challenges associated with conventional high-capacity anodes like silicon or lithium metal. Compared to HE-oxides, there is little research regarding HE-alloys for anode materials. Although it is unlikely that entropy stabilization would eliminate the swelling associated with Li alloying, HEAs may reduce capacity fade in otherwise high-capacity anodes. Researchers should consider strategies such as the combination of alloying (e.g., Sn, Al) and inert/non-alloying elements (e.g., Cu), and the use of elements with a large atomic radius (e.g., La). The practical and economic feasibility of the latter option is increased if the composition is not limited to strict equiatomic proportions.

In recent years, high-entropy materials have been employed in numerous fields of energy research, but more work needs to be done. Given the complexity of these materials, it is expected that computational techniques and high-throughput experimentation will play a key role in the future of the material design cycle. However, this outlook should be approached critically. Computational design strategies arise out of pre-existing paradigms and may overlook truly novel materials. For instance, high-throughput studies on HE-rock-salts may direct resources away from the discovery of entirely new families of HE structures. Additional challenges are related to the collection of high-quality data (a prerequisite for the successful implementation of machine learning). Ideally, high-throughput experimentation will be used in conjunction with computational designs and the more traditional experimental exploration.