Next-gen battery trends

From pv magazine print edition 3/24

Innovation at material, cell, and system level has been just as important in lithium ion’s leap forward as supply chain development. Stationary energy storage cell design is trending toward large-format prismatic cells thanks to lithium ferro-phosphate (LFP) battery chemistry. Lower costs, a longer cycle life, and better safety have seen LFP batteries eat into nickel-manganese-cobalt (NMC) market share since 2020. The energy density and charge rate offered by NMC devices has instead seen them favored for mobility uses.

New materials

Both offer room for development. Reducing cobalt and raising nickel content in NMC batteries offers lower costs and better energy density. LFP energy density can be improved by replacing some iron cathode material with manganese – for an LMFP cathode. This emerging technology shows great promise, as it provides roughly 15% to 20% more energy density, or up to 230 Wh/kg, while maintaining the same level of cost and safety as LFP batteries. Financial services firm Ernst & Young calculates that the costs of LMFP batteries are about 21% higher than that of LFP devices on a dollars-per-kilogram basis. Considering their higher energy density, however, the cost per watt-hour is 5% lower, making them much more economical.

Although LMFP batteries offer higher voltage and energy density than LFP, there are trade-offs. “There are issues with lower power capability and lower cycle life which arise from the lower electrical conductivity of LMFP compared to LFP, which limits the power capability; and the dissolution of the manganese into the electrolyte over cycling, which limits cycle life,” said John-Joseph Marie, energy storage analyst at British research body The Faraday Institution. As a result, Marie doesn’t see LMFP replacing LFP for short-duration storage. “If the issue of cycle life could be solved, LMFP could be a cost-efficient technology for longer storage durations, e.g. four to eight hours,” he said.

Moves toward mass production of LMFP batteries are picking up pace, though – especially in China. CATL, Eve, BYD, and Gotion – as well as South Korea’s Samsung SDI and United States-based Mitra Chem – are now at various stages of commercialization and are producing this technology. In mid 2023, Gotion High-Tech set a precedent for developing NMC-free batteries with a range that reached 1,000 km with the launch of its L600 Astroinno battery cell and pack, featuring LMFP chemistry. The Chinese manufacturer said its new battery technology, which has undergone a research period of 10 years, is scheduled to begin mass production in 2024.

On the anode side, graphite remains the go-to material with efforts made to boost its lithium-holding capacity by adding a small amount of silicon. The addition of silicon is an attractive proposition, as it would allow for almost 10 times more capacity, due to its theoretical capacity of 3,600 milliampere-hours per gram (mAh/g), compared to graphite’s maximum of 372 mAh/g. In 2023, Tesla reportedly added up to 5% of silicon to its graphite anodes via intermetallic alloying. Startups are even more bullish on the technology, with a few of them proposing 100% silicon anodes.

The advantage on the cost side is also pronounced. “Silicon anodes, including silicon composite anodes and pure silicon anodes, are expected to be cheaper to produce on a dollar-per-kilowatt-hour basis than the incumbent graphite anode technology, thanks to their much higher capacity,” said Marie.

London-based consultants Rho Motion have modeled the cost pathway for lithium ion cells using various evolutions of silicon anode up to 2030. The results showed that the current anode active material accounts for around 7% to 9% of total cell costs in NMC811 (80% nickel, 10% manganese, and 10% cobalt) and LFP chemistries, a figure which is expected to reduce to around 2% by 2030 with the adoption of optimized microsilicon at scale.

Silicon anodes are set to increase their market share. Analyst BloombergNEF says anode technology could lean on silicon, lithium, and hard carbon to displace 46% of graphite demand in 2035, compared to scenarios in which the market doesn’t shift away from graphite. While the analysts expect hard carbon used in sodium ion batteries to already start entering the market in 2024, lithium metal anodes are projected to start playing a more prominent role only beyond 2030.

Bigger, better

As the search for new battery materials continues, so does innovation on the cell level. In battery energy storage system (BESS) applications, however, cost remains the key driver for adjustments at cell level. The obvious way of bringing down bill-of-materials (BOM) costs is by increasing the capacity and size of cells. Another benefit is that fewer cells means less work for the battery energy management system. Finally, the number of connection points and the complexity of mechanical trays are also reduced, making manufacturing processes easier.

Several manufacturers have already transferred from 280 ampere-hour (Ah) to 300 Ah-plus cells, with larger capacities in the pipeline. With the former seen as a standard in utility scale storage projects in 2023, the 300 Ah-plus cells stand ready to power commercial projects in 2024. Christoph Neef, senior scientist and project manager at Germany’s Fraunhofer Institute for Systems and Innovation Research ISI, observed that the trend toward high-capacity cells does not come from the electric vehicle (EV) industry, where only cells up to 200 Ah are usually deployed.

“The reason is simple: the system sizes are rather small compared to industrial BESS’, e.g. only 70 kWh,” said Neef. “Nevertheless, a system voltage of more than 300 V or, in future, 800 V should be achieved. This is simply not possible with a few high-capacity cells connected in series. The effects of the failure of a single cell would also be incalculably high.”

In other words, 300 Ah-plus cells are a clear indication battery cell manufacturers are increasingly developing products tailored to the BESS market and slowly moving away from the dominance of EV-customized cells.

Most cell manufacturers offer 300 Ah-plus cells with the same dimensions as 280 Ah devices, which makes the job easier for system integrators when designing their products. Large cells pose a greater challenge for safety management, however – particularly in terms of ensuring temperature uniformity within cells.

“The larger the individual cell, the greater the impact if a cell fails,” said Neef. “However, we do not assume that this has any effect on the fire risk.” Large-format cells, with more than 300 Ah with LFP chemistry, are likely to exhibit similar behavior in the event of a short-circuit as smaller units. The impact on the long-term reliability of the system could be higher, however. Precise cell monitoring and predictive maintenance are becoming increasingly important as a result.

“What is very interesting, however, is that the production quality of cell manufacturers is now so high that large-format cells are also worthwhile from the point of view of rejects,” added Neef. “The larger the cell capacity, the higher the risk of defects, e.g. in the electrode, and therefore the higher the risk of producing poor-quality cells. Only with very low defect rates is the production of these large cells worthwhile. [It is] a sign of the maturity of the industry.”

System evolution

Such bigger cells have led to bigger capacities at the container level. Average capacities have moved from more than 3 MWh to more than 5 MWh for the same 20-foot-equivalent units. Such higher energy-density BESS’ come with a host of advantages. For instance, they make it possible to install bigger capacities on a smaller footprint and thus save on land use – a particularly important consideration in regions facing space constraints.

“Less number of cells per watt-hour also reduces assembly and packaging and saves cost during production processes,” said Anqi Shi, senior analyst for batteries and energy storage at S&P Global. “However, challenges occur in system design and safety measures as heat dissipation and consistent temperature is harder to achieve.”

Another pronounced development at the system level is the move from air cooling to liquid cooling. In 2023, many system integrators released new BESS products with liquid cooling, describing it as an upgrade in terms of efficiency and the lifespan of grid scale products. Compared to the previously ubiquitous air cooling method, liquid cooling is better geared to deal with temperature-related challenges and is able to reduce land footprint, with some manufacturers reporting saving more than 20% of floor area.

That technology uptake has been particularly pronounced in China, where the lion’s share of large scale energy storage project tenders require liquid cooling technology. Shi said liquid cooling is especially suitable for high energy-density BESS’, or for projects where fast charging and discharging, and wide ambient temperature changes, are expected. “However, while liquid cooling offers better heat dissipation efficiency and temperature uniformity inside the BESS, the cost is higher than air cooling and there could be a risk of coolant leakage,” added Shi.

Finally, a potential new trend could be the emergence of alternating-current, “AC blocks” – containers with both battery and power conversion systems (PCS’) integrated inside. That type of product was pioneered by Tesla, in the company’s 2022 Megapack redesign. “AC blocks offer easier and quicker installation on site, less land occupation, and decentralized (pack-level or rack-level) control of BESS’,” said Shi. “They come at a cost premium, mostly due to using string PCS’, but the cost could be reduced by scaling up production.”

Currently, only Sungrow and Tesla have AC blocks that are larger than 3 MWh in a single container. Other relatively newer players, including Rimac Energy and JD Energy, have less-than-1 MWh AC block containers which might not have the same land-use advantage. It is not yet clear whether AC blocks can become a mainstream trend. After all, not all system integrators have in-house PCS capability. As Shi noted, however, AC blocks give developers increased choice and further squeeze established players in an already fiercely competitive market.