The rechargeable battery market is dominated by lithium-ion batteries (LIBs). They are everywhere, powering our mobile phones and the current generation of consumer electric vehicles. Small, light lithium ions act as charge carriers, shuttling back and forth across the LIB during charge and discharge, allowing for high energy densities. Even the next-generation solid-state batteries expected to hit the market soon rely on lithium[1]. There are, however, other types of metal-ion battery in play. Sodium-ion batteries (SIBs) have been known for decades, having essentially the same operating principle as LIBs but replacing lithium with heavier sodium. This increase in weight leads to lower energy densities than LIBs, but SIBs are now receiving commercial interest due to safety and cost advantages[2]. The SIB charge is being led by parties big and small, from the gigafactory-sized giants Northvolt and CATL, to startups such as Faradion and Natron Energy.
Key components of a metal-ion battery are the anode and cathode active materials which store and release metal ions during charge and discharge. Part of the cost advantages of SIBs are achieved because active materials which work well with sodium ions can be prepared from cheap and abundant precursors. Multiple parties are developing SIBs using hard carbon[3] as the anode material and Prussian blue[4] analogs (PBAs) as the cathode material. Hard carbon has been known for use in metal-ion batteries since the 1990s. PBAs were proposed for use in SIBs by John Goodenough and colleagues from the University of Texas in 2012 (this is just one of Goodenough’s many contributions to battery chemistry over his long career; for more information read our retrospective on his patents).
Patenting the current generation of active materials
It is common to seek patent protection for active materials. A spate of US patents[5] originating from Goodenough’s team were filed in 2012 aimed at protecting various applications of PBAs to SIBs, including a pairing of a manganese-containing PBA cathode material with a hard carbon anode material. Given that the direction of travel for SIBs does seem to be based on PBAs and hard carbon, obtaining new generations of patent protection for these materials will likely be a lively battleground for innovators and their attorneys. The established use of PBAs and hard carbon will make this challenging, but there should still be plenty of room to find patentable subject-matter for these materials.
In particular, for a relatively mature field such as SIBs, where the fundamental chemistry of key materials has been established and perhaps already patented, subsequent generations of patent filings can focus on performance optimisation by controlling the physical parameters of the materials. For instance, it could be envisaged that a PBA having a certain particle size distribution, surface area, and/or sphericity might provide an unexpected improvement over the state of the art. Controlling the porosity of a hard carbon anode material may reveal another unexpected improvement. Specifying the parameters which provide these improvements – a parameter sweet spot – could represent patentable subject-matter[6].
However, relying on parameters for patentability brings problems of its own. Some patent offices, in particular the European Patent Office (EPO), adopt a very strict assessment of the clarity of parameters, typically requiring that a clear and complete measurement method for each parameter must be specified in the claims. If a suitable measurement method was not included when the application was drafted then, in the extreme, such objections could lead to the refusal of an otherwise allowable application. Thus, great care should be taken during the drafting stages to include sufficient detail on measurement methods for parameters (see our article for further information).
Another area for attention during drafting is to include sufficient experimental evidence to prove that the claimed parameter sweet spot is indeed as sweet as the inventor says that it is. Ideally, this requires not just datapoints showing that good performance exists when inside that sweet spot, but also datapoints showing that there is inferior performance when outside that sweet spot. Accordingly, having a robust set of comparative examples representing the unclaimed state of the art can be just as important as inventive examples representing the claimed subject-matter. Good communication between the technical team generating the data and the legal team drafting the patent application is essential in drafting a suitable set of examples.
Patenting further active materials
Active materials other than hard carbon and PBAs are envisaged for use in SIBs. However, gaining patent protection for other active materials may not be straightforward due to the shadow cast by LIBs. Due to the size and growth of the market, LIBs have been the subject of a great deal of R&D in the past 30 years. Naturally, this has been accompanied by a large number of patent filings attempting to protect a wide variety of LIB active materials. Such patent applications, although likely drafted with the ultimate goal of obtaining protection in the field of LIBs, are routinely drafted to be broad enough to protect the use of the active materials in SIBs – typically even explicitly referring to SIBs. This is a reasonable approach from the LIB innovator (the author has drafted many such patent applications!), as a patent protects all uses of a patented material. The consequence of this drafting practice is that the numerous LIB patent applications represent a deep pool of prior art disclosing a wide variety of active materials explicitly for use in SIBs, even though that prior art was likely based entirely on research carried out on LIBs, never considering SIBs beyond a hypothetical level. If one such active material is subsequently found to be particularly effective for use in SIBs it may be then difficult to patent – an outcome which seems rather unfair. However, with careful drafting, all hope is not lost for gaining patent protection, even for an active material already disclosed for use in SIBs in an LIB patent application.
One strategy is to consider what desirable SIB performance parameter is achieved when using the active material, and to specify that performance parameter in the claims. For instance, if a desirable energy density, cycle life, and/or rate performance is achieved via the active material, a claim could be drafted to an SIB comprising the active material and requiring that performance. It could reasonably be argued that the achievement of that performance in an SIB would not have been obvious based on a solely hypothetical disclosure of the active material in an SIB. However, and as with any parameter, care should be taken when drafting the patent application to include sufficient detail on how the performance parameter is to be measured, to minimise the risk of the clarity objections discussed above.
Another point to keep in mind if following this strategy is that it may be necessary to specify the other key components of the battery which contribute to the achievement of the claimed performance parameter. At the EPO, claims requiring the achievement of a desirable parameter often receive clarity objections that the claims must exhaustively specify all the technical features which are needed to achieve that parameter (known as a result to be achieved objection). For example, if the invention resides in the discovery of a particular anode material for an SIB, which is narrowly defined in the claims along with a performance parameter, some EPO examiners may nonetheless insist that the cathode material is also specified in the claims. Although such objections can be overcome by argument, it is important when drafting a patent application relying on a performance parameter to include plenty of fallback positions for amendments to further define the other components of the SIB, as this may be needed should the examiner refuse to give ground.
Of course, an alternative approach to obtaining patent protection for SIB active materials is to find a new class of material not considered before for use in metal-ion batteries – ground untrodden even by the breadth of LIB research. The scope of the periodic table and the numerous crystal structures suitable for sodium-ion intercalation should still leave plenty of room for innovation here.
Summary
In summary, innovators may face challenges in gaining new generations of patent protection for SIBs at the stage when they are reaching commercialisation. However, with care and attention at the drafting stages, including good communication between the technical and legal teams, such challenges should not be insurmountable.
