Battery Materials Synthesis
NREL's development of inexpensive, high-energy-density electrode materials is challenging but critical to the success of electric-drive vehicle (EDV) batteries.
The greater energy and power requirements and system integration demands of EDVs pose significant challenges to energy storage technologies. Making these materials durable enough that batteries last more than 10 years is essential. These materials go through thousands of charge/discharge cycles, while being exposed to the harsh chemical, thermal, and mechanical environment found inside a battery cell. NREL researchers work hand-in-hand with industry partners to address these challenges with new materials and processes for a full range of batteries designed to power tomorrow's energy-efficient vehicles.
Lithium-ion (Li-ion) batteries have become automakers' preferred EDV energy storage option, capable of delivering the energy and power density required by hybrids (HEVs), plug-in hybrids (PHEVs), and all-electric vehicles (EVs) in a relatively small, lightweight package. While Li-ion-equipped vehicles are already rolling off car lots, NREL researchers continue to explore refinements and new options, such as lithium-air, magnesium-ion, and solid-state technologies. NREL's energy storage materials research concentrates primarily on the composition and coating of electrodes as well as thermal interface materials including greases, phase-change materials, thermoplastics, and graphite to maximize battery performance.
Unstable interphases and surface side-reactions between organic electrolytes and electrode surfaces can trigger interface instability and durability problems in batteries. These issues typically shorten battery lifespan and diminish reliability.
NREL research has achieved greater battery stability through both conventional and innovative methods. The lab's introduction of metal oxide and hybrid inorganic-organic surface modification via atomic layer deposition has provided innovative and cost-effective methods to mitigate lifespan and reliability concerns.
Atomic Layer Deposition
NREL and its partners have developed a breakthrough method for applying coatings directly on as-formed composite electrodes using atomic layer deposition (ALD). ALD is the current state-of-the-art method for applying conformal thin film coatings to highly textured surfaces. These coatings have been shown to enhance cycle life and abuse tolerance in Li-ion batteries. Improvements in performance can be traced back to mitigation of deleterious side reactions and prevention of mechanical degradation. These coatings must be optimized to match electrode material and thickness.
Building on the success of optimized electrode coatings in improving Li-ion battery performance, NREL is working with university collaborators to develop a new electrode coating method that transfers the ALD process into an in-line, roll-to-roll format that can be integrated with manufacturing methods.
Significant advances in battery energy density and rate capability are needed for electric-drive vehicles to offer the reliability, durability, and safety demanded by a larger market. Materials with high energy densities often fracture, degrade, and rapidly lose capacity due to expansion and contraction when the battery is charged or discharged at a high rate. NREL has managed to increase battery lifespan, rate capability, capacity, and safety through the development of novel nanostructured electrode materials.
Scientists at NREL have created crystalline nanotubes and nanorods to address Li-ion battery thermal management, weight, and conductivity issues. NREL's high-performance, binder-free, carbon-nanotube-based electrodes can optimize battery charging, while reducing swelling and shrinking that can shorten electrode lifespan. An array of custom-built apparatus makes it possible for NREL to conduct nanostructured synthesis research.
Metal Oxide Anodes
Transition metal oxides are capable of a significantly larger reversible capacity than commercial-grade graphite. Molybdenum oxide can produce a stable capacity nearly three times that of conventional graphite anodes. Iron oxide is among the most abundant and least expensive elements, and can outperform many other materials when nanoparticles are used in electrodes produced with NREL's innovative fabrication techniques.
NREL is working in partnership with other national labs via the Silicon Anode Consortium to investigate the use of silicon as a possible viable alternative to graphitic carbon as an anode material. Because of its high capacity and availability, silicon has the potential to improve energy density and reduce costs. However, several issues may limit its utility, including swelling upon lithiation (which can lead to particle cracking, particle isolation, and electrode delamination) and electrolyte side reactions (which can affect cycling efficiency). Additionally, understanding the mechanisms of solid electrolyte interphase formation and failure is necessary to enable functioning silicon anodes in Li-ion batteries.
Critical-Material-Free Battery Technologies
As the demand for electric vehicle fast charging at levels of 350 kilowatts or higher continues to rise, new approaches are needed to avoid considerable cost and grid-resiliency impacts. Innovative critical-material-free behind-the-meter energy storage solutions addressing these issues could apply to other short-duration, high-power-demand electric loads as well.
Behind-the-Meter Energy Storage
NREL is working in partnership with other national labs via the Behind-the-Meter Storage Consortium to develop novel, critical-material-free battery technologies ultimately supporting the integration of electric vehicle fast chargers, photovoltaic generation, stationary energy storage, building systems, and the electric grid. Research targets the development of innovative energy storage technologies for stationary applications below 10 megawatt-hours to minimize the need for significant upgrades to the electric grid. Such technologies could also eliminate excess demand charges that fast charging would incur using current technologies.