Electrifying the future: The contenders aiming to topple Lithium-Ion

As the world shifts from fossil fuel dependence to an electrified future, the success of our transition to clean energy lies not just in how we generate power, but in how we store it. The global battery landscape is moving beyond the dominance of traditional lithium-ion (Li-ion) batteries into a new era, where sodium-ion (Na-ion), lithium-sulphur (Li-S) and solid-state batteries are on their way to becoming industrial mainstays.

In this article, we explore some of the technologies aiming to replace Li-ion across a range of applications.

Sodium-ion: The abundant alternative

Na-ion batteries have long been considered the primary alternative to Li-ion technology, though research effectively stalled once Sony successfully commercialised the Li-ion battery in 1991 ^[1]. It wasn’t until the early 2010s, when concerns arose about the scarcity of lithium and the environmental impact of mining it, that research into alternatives began again in earnest.

Na-ion batteries operate on the same fundamental “rocking-chair” principle as Li-ion ones, namely shuttling ions back and forth through an electrolyte between an anode and a cathode. Importantly, however, they substitute scarce, costly and environmentally damaging lithium for sodium, the sixth most abundant element on Earth. This shift moves the supply chain away from geographically concentrated lithium deposits toward a resource that can be obtained essentially anywhere there is salt.

Aside from the ecological benefits, Na-ion batteries present several advantages over Li-ion ones. Sodium is significantly cheaper than lithium. Na-ion cells also exhibit greater thermal stability, making the batteries less prone to the thermal runaway fires associated with lithium, and they also perform better in the cold. These characteristics make Na‑ion systems particularly well‑suited for stationary grid storage and other applications where safety, cost and temperature resilience are critical.

However, Na-ion technology also has its limitations. Because sodium ions are heavier and larger than lithium ions, Na-ion batteries have a lower energy density than Li-ion ones. Therefore, they need to be heavier and larger to hold the same charge. They also currently have a shorter lifespan than the highest-grade Li-ion cells.

Historically, these constraints have limited Na-ion batteries to stationary applications, but that is finally beginning to change. Several car manufacturers are now looking to implement Na-ion into smaller, urban cars, where shorter driving ranges are an acceptable trade-off for a lower cost and superior cold-weather performance. A range of sodium-powered electric vehicles (EVs) are expected to launch over the next couple of years, signalling a new era of battery diversity in the EV market.

Solving the Anode Challenge

One of the primary technical hurdles for Na-ion batteries was the anode. Lithium ions intercalate efficiently into graphite (the standard anode material). In contrast, sodium ions are too large for the interlayer spacing of graphite, leading to structural degradation of the anode as they intercalate. The breakthrough came with the development of hard carbon anodes. Unlike the neat stacking structure of graphite, hard carbon has large, irregular voids that accommodate the sodium ions. Hard carbon has been known as an anode material for decades, but research is still underway to improve this anode material, with recent patent applications focusing on the structure and composition of the anode, as well as different hard carbon precursors.

Lithium-Sulphur: pushing the density limit

Lithium–Sulphur (Li‑S) batteries were also developed in response to key limitations of conventional Li‑ion batteries, such as cost and resource constraints. Na-ion improves cost and safety at the expense of size and weight. However, Li‑S targets the opposite end of the performance spectrum, namely maximum energy density for weight‑sensitive applications, such as long‑range EVs, drones and aerospace.

In Li-S batteries, the transition metal-based cathodes used in Li-ion and many Na-ion batteries are replaced with sulphur, avoiding many of the geopolitical complexities associated with the cobalt and nickel in Li‑ion and Na-ion supply chains. Sulphur confers the same kind of resource and cost benefits demonstrated by sodium. It is an abundant, non-toxic byproduct of oil refining, making it cheap and environmentally friendly compared to the transition metals.

Li‑S technology relies on a multi-electron redox reaction between a lithium anode and a sulphur cathode. During discharge, the lithium atoms are oxidised and the sulphur undergoes a series of complex chemical transitions to form lithium sulphides. This process allows for a much higher theoretical energy density than conventional intercalation chemistry. This positions Li‑S as a high‑performance counterpart to Na‑ion’s low‑cost, robust and cold‑tolerant profile. However, several constraints have kept Li-S batteries in the lab and out of the mass market.

In particular, the sulphur cathode undergoes a volume expansion of up to about 80% during discharge. The physical stresses resulting from this expansion can cause the battery to degrade or fail after only a few hundred cycles. Li‑S batteries also suffer from the formation of lithium dendrites. Unlike in lithium-ion batteries, where the graphite anode provides a host structure for the lithium, Li-S technology requires that the lithium plates and strips directly from the metal surface of the anode. Dendrites are tiny, needle-like metallic growths on the lithium anode that form naturally during such plating. Reactions with intermediate sulphur compounds (polysulfides) and uneven lithium deposition also create non‑uniform current distributions, making dendrite formation even more likely. Another hurdle to overcome is the polysulfide “shuttle” effect, where polysulfides dissolve into the electrolyte and migrate back and forth between the electrodes, leading to rapid capacity loss and a short cycle life.

One approach to try and mitigate the issues with Li-S batteries involves using a cathode comprising layers of agglomerates of porous carbon nanoparticles having regions of different porosity. The nanoparticles can confine polysulfides and the agglomerates can mitigate the effect of mechanical stresses by accommodating expansion. In addition, a “hybrid” electrolyte system is provided, which includes a liquid electrolyte and a polymer electrolyte confined in the porous carbon agglomerates to physically trap polysulfides at the cathode. This effectively kills the shuttle effect and drastically improves the lifetime of Li-S batteries. Moreover, since the electrolyte is a solid barrier, it is much harder for lithium dendrites to pierce through the electrolyte and cause a short circuit.

This type of hybrid, or semi-solid-state, system bridges the gap between traditional liquid electrolytes and solid-state batteries, which were long considered the holy grail of battery technology.

The solid-state frontier

Solid-state batteries (SSBs) work by replacing the liquid electrolyte with a solid electrolyte (such as a ceramic or a polymer) between a metallic lithium anode and a cathode formed of an oxide or sulphide. Solid-state materials are non-flammable and lack the volatile organic solvents found in liquid electrolytes. This means SSBs can operate at higher temperatures than Li-ion, without the risk of thermal runaway or fire.

Despite these benefits, SSBs present a new set of hurdles. Because both the cathode and the electrolyte are solids, there can be poor interfacial contact between the two materials. Some solid electrolytes can also react with the cathode material, forming resistive layers. As such, SSBs can have a high interfacial resistance which makes it difficult for the lithium ions to move across the boundary. This often leads to slower charging speeds and lower power output than Li-ion batteries. This interfacial resistance is a major area of current research.

An alternative approach to SSBs has been to build a solid-state device based on sodium, rather than lithium^[2]. A solid-state sodium-metal-chloride battery has been developed, in which sodium ions from the sodium-metal-chloride cathode travel through a very thin (micrometre-scale) sodium-ion conducting ceramic electrolyte. Advantageously, the combination of a sodium-metal-chloride and a sodium-conducting ceramic electrolyte successfully bypasses the need for liquid components, which eliminates the risk of fire. Since the sodium must be molten to achieve the necessary conductivity through the ceramic electrolyte, these batteries operate efficiently in extreme heat and do not need cooling systems (they are designed to operate at 250°C). The technology is specifically engineered for long-term stationary energy storage, making them a welcome accompaniment to renewable energy sources, such as solar and wind.

Future prospects

China is leading the global scale up of Na-ion so far, but the UK is aiming to be at the forefront of this new era of battery technology with their Battery Innovation Programme (BIP), delivered by Innovate UK. The BIP represents a massive £452 million investment “designed to accelerate battery research, innovation and manufacturing scale-up, securing the UK’s global leadership in clean energy and advanced manufacturing to 2030 and beyond”^[3].

As these contenders move toward mass production, the patent landscape is becoming increasingly crowded, with filings at the EPO in the electrical machinery, apparatus, energy category (which includes battery technology) increasing 36% between 2021 and 2024. This surge in filings can lead to risks of patent thickets, where multiple overlapping patents make it difficult to enter the market. If you are interested in freedom to operate or patent protection for any aspect of your battery technology, please get in touch with Alexandra Collingwood-Pierce or your usual Venner Shipley adviser.

In future articles, we will take a look at some of the other energy storage solutions available, as well as discussing how close we are to making batteries fully sustainable and recyclable, another goal of the BIP.