Advanced Battery Technologies for Stationary Energy Storage Applications: Page 3 of 5

NMC-type Li-ion. In 2001, Zhonga Lu, a staff scientist for 3M, and Jeff Dahn, a physics and chemistry professor at Dalhousie University, filed international patents for lithium nickel manganese cobalt oxide (LiNiMnCoO2) as a cathode material. As an industrial research chair for the Natural Sciences and Engineering Research Council of Canada, Dahn specializes in materials for advanced batteries. After completing a 20-year research agreement with 3M in 2016, Dahn’s group began a 5-year research partnership with Tesla.

Cobalt is a common ingredient in many Li-ion battery cathodes because it provides a high energy density. The downside is that cobalt is expensive, and the cobalt-based chemistries used for portable electronics have issues with thermal stability and capacity fade. Adding nickel and magnesium to the mix not only reduces costs, because nickel is less expensive than cobalt, but also improves thermal stability and cycle life. Li-ion batteries with NMC-type cathodes are increasingly popular in the market because they offer good all-around performance—in terms of cost, safety and lifespan—and can be tailored for energy or power applications. In its 2017 report, “Status of the Rechargeable Li-ion Battery Industry,” Yole Développement predicts that NMC materials will account for more than half of the global cathode market by 2022.

Notable grid-interactive NMC battery vendors include LG Chem; Kokam, a South Korean company that specializes in rechargeable Li-ion polymer batteries; Panasonic, which is Tesla’s primary business partner in its much-anticipated Gigafactory; and Samsung. Kokam’s Li-ion rack system, which is integral to its containerized energy storage systems, provides a good example of the flexibility of NMC batteries. With two battery racks in parallel, Kokam’s high power–type NMC battery has a usable energy capacity of 211 kWh and a power rating of 888 kW when charging or discharging. By contrast, the same configuration of high energy–type NMC batteries provides 17% more usable energy (253 kWh), but only one-third as much power (266 kW). As a rule of thumb, high power–type NMC batteries are intended for short-duration (<1 hour) applications that require the rapid dispatch of large amounts of power; energy-type NMC batteries are intended for longer-duration (>1 hour) applications with more-continuous loads.

Project developers, utilities, EPCs and system integrators are deploying NMC-type batteries at every level of the electric power system, from grid-scale to customer-sited applications. For example, Tesla deployed a massive 20 MW/80 MWh energy storage system, consisting of roughly 400 Powerpack 2s, at the Southern California Edison (SCE) Mira Loma substation. SCE will use the project to store low-cost off-peak energy for dispatch as a means of reducing its reliance on natural gas peaker plants. At the other end of the size spectrum, Tesla and Green Mountain Power are providing Tesla’s Powerwall 2 to residential customers in Vermont for a low monthly lease or up-front purchase price. Tesla is working with Green Mountain Power to aggregate these distributed residential energy storage resources and bundle them with Powerpack deployments located on utility-owned property for dispatch as a virtual power plant that can provide a variety of grid services.

According to GTM Research, flow batteries accounted for just 5% of the US energy storage market in Q2 2017. As the technology matures, flow batteries could capture more market share in niche applications. While most analysts believe it will be difficult to displace Li-ion batteries, at least over the short term, in applications with a sub–4-hour discharge duration, many think that an opportunity exists for new technologies in applications where discharge duration is on the order of 4–6 hours or more. Examples of applications that favor a long-discharge battery include electric energy time shift or energy arbitrage, transmission and distribution upgrade deferral, and microgrids with variable renewable resources.

Flow batteries are constructed and scale up very differently from Li-ion batteries. Most flow batteries consist of two liquid electrolyte tanks and a cell stack area, like the vanadium-based flow battery in Figure 2. During operation, pumps circulate the electrolyte materials through porous electrodes in the cell stacks, which an ion-specific membrane separates to allow electron exchange between the positively charged catholyte and the negatively charged anolyte. The cell stack structure of this battery type is similar to that of a fuel cell, except that a secondary flow battery can re-energize and reuse the electrolyte.

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