The development of lithium-ion batteries in the 1970s and 1980s involved extensive research into cathode materials before lithium cobalt oxide (LiCoO₂) emerged as the dominant choice. Early investigations explored a variety of transition metal chalcogenides, phosphates, and other oxides, each with distinct electrochemical properties and limitations. These efforts were critical in shaping the understanding of intercalation chemistry and the structural requirements for high-performance cathodes.

One of the earliest classes of materials studied were transition metal chalcogenides, particularly titanium disulfide (TiS₂). This material demonstrated promising intercalation properties due to its layered structure, allowing lithium ions to reversibly insert between the sulfide layers. TiS₂ exhibited a theoretical capacity of approximately 240 mAh/g and operated at a voltage of around 2.1 V versus lithium. However, its relatively low energy density and the tendency for polysulfide dissolution in organic electrolytes limited its commercial viability. Additionally, the low operating voltage reduced the overall energy output compared to higher-voltage alternatives.

Vanadium oxide (V₂O₅) was another candidate investigated for its high theoretical capacity and ability to intercalate multiple lithium ions. The orthorhombic phase of V₂O₅ could accommodate up to three lithium ions per formula unit, translating to a capacity exceeding 440 mAh/g. Despite this advantage, structural instability during cycling led to rapid capacity fade. The material underwent irreversible phase transitions upon deep discharge, causing mechanical degradation and poor cycle life. Researchers also noted that the high lithium content caused excessive volume changes, further exacerbating structural breakdown.

Molybdenum disulfide (MoS₂) and other dichalcogenides were examined for their layered structures similar to TiS₂. MoS₂ showed moderate electrochemical activity, but its capacity was limited by sluggish lithium diffusion kinetics. The strong covalent bonding within the MoS₂ layers hindered rapid ion transport, resulting in lower rate capability compared to oxides. Additionally, the material’s operating voltage was insufficient to compete with emerging oxide-based cathodes.

Transition metal phosphates, such as iron phosphate (FePO₄), were explored due to their excellent thermal and chemical stability. The olivine structure of FePO₄ provided a rigid framework that minimized structural degradation during cycling. However, the insulating nature of the material led to extremely poor electronic conductivity, severely limiting its rate performance. Early attempts to mitigate this issue through carbon coating or particle size reduction were only partially successful, and the low operating voltage of around 3.4 V versus lithium made it less attractive than higher-voltage alternatives.

Manganese dioxide (MnO₂) in various crystallographic forms was also investigated, particularly for its low cost and environmental friendliness. The spinel structure of LiMn₂O₄ later became significant, but early studies focused on non-lithiated MnO₂ polymorphs. These materials suffered from poor reversibility due to structural rearrangements during lithium insertion. The tunnel structures of some MnO₂ variants allowed lithium intercalation but often collapsed upon repeated cycling, leading to rapid capacity loss.

Nickel oxide (NiO) and cobalt oxide (CoO) were examined as potential cathodes, but their rock-salt structures lacked the layered or open frameworks needed for efficient lithium intercalation. These materials exhibited limited capacity and poor cyclability due to irreversible phase changes. However, their investigation provided valuable insights into the role of transition metal redox chemistry, which later informed the development of layered oxides.

The limitations of these early cathode materials highlighted several key requirements for viable lithium-ion battery electrodes. First, the host structure needed to maintain stability during repeated lithium insertion and extraction. Materials that underwent significant volume changes or phase transitions tended to degrade quickly. Second, electronic and ionic conductivity were critical for achieving high rate capability. Insulating materials, even with high theoretical capacities, were impractical without modifications to enhance charge transport. Third, the operating voltage needed to balance energy density with electrolyte stability. Too low a voltage reduced energy output, while too high a risked electrolyte decomposition.

These findings steered researchers toward layered transition metal oxides, particularly LiCoO₂, which addressed many of these challenges. The layered structure of LiCoO₂ allowed for reversible lithium intercalation with minimal structural disruption. Its operating voltage of around 3.9 V versus lithium provided a favorable balance between energy density and electrolyte stability. Additionally, cobalt’s electronic configuration enabled good electronic conductivity, reducing the need for extensive conductive additives.

The success of LiCoO₂ also underscored the importance of transition metal redox activity in cathode materials. The ability of cobalt to reversibly change oxidation states during cycling was crucial for maintaining capacity over many cycles. This principle later guided the development of alternative layered oxides, such as lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum oxide (NCA), which built upon the foundational work of early cathode research.

In summary, the exploration of alternative cathode materials during the early development of lithium-ion batteries was instrumental in identifying the structural and electrochemical requirements for high-performance electrodes. While chalcogenides, phosphates, and other oxides exhibited specific advantages, their limitations in stability, conductivity, and voltage led researchers to favor layered oxide structures. This knowledge not only solidified LiCoO₂ as the first commercially successful cathode but also laid the groundwork for ongoing advancements in lithium-ion battery technology.