Lithium ion (Li-ion) batteries have quickly become the preferred battery technology for most applications. Various forms of Li-ion exist with cells and battery packs designed for a range of applications from high power to high energy density all in a very cost effective package. Its advantages in energy density by weight and volume are well known. However, the additional benefits of Li-ion such as long cycle life, shorter charge times, the ability to fuel gauge and predict end of life accurately, and significant improvements in the long term stability at elevated temperatures have accelerated Lithium ion’s adoption into even more applications particularly those previously serviced by older chemistries such as Nickel Metal Hydride or Lead Acid.
Mid-rate cells are a new class of primarily 18650s that have been developed recently to address primarily electric bike applications. These cells are compromise between discharge rate and capacity with added design target of improving low temperature performance. The capacities range typically from 2.0-2.4Ah with discharge rates typically ranging 3-5C.
High-rate cells sometimes also referred to as ‘power tool’ cells are designed to perform very high discharge rates. These cells are designed to provide up to 30C rate discharges with the tradeoff being reduced capacity typically from 1.1-1.7Ah in an 18650 size.
High-Energy Density Cells
The High-Energy Density cells are the traditional Li-ion cells that have been optimized for highest capacity possible. These are the cells used in applications where runtime is the top priority. These cells are typically limited to 1-2C discharge rates with 18650 capacities up to 3.1Ah now readily available.
Lithium polymer has a wide range of performance encompassing all of the various types of lithium ion mentioned above. There are low to high rate cells available with capacities generally from as little as 20mAh up to 4000mAh. Lithium polymer batteries (Li-polymer) offer several advantages. Li-polymer has a greater energy density in terms of weight than lithium ion. In very thin cells (under 5mm) lithium polymer also provides higher volumetric energy density. Generally there is more flexibility in cell sizes and shape with Li-polymer and with high quality polymer cells offering superior stability in over-voltage and high temperature conditions. Many smart phones, GPS devices and laptops now employ lithium polymer cells.
Lithium Iron Phosphate
While still in the class of Li-ion cells, lithium iron phosphate chemistry does have many unique characteristics. These are now fundamentally designed as high rate cells. Lithium iron phosphate cells are capable of up to 30-40C rate discharges in some cases. The energy density is significantly lower than conventional high rate Li-ion cells with a typical 18650 capacity of 1.0Ah and a nominal voltage of 3.2-3.3V. Lithium iron phosphate is distinguished by its long cycle life 1000-4000 cycles depending on the regime and by the safer characteristics of the iron phosphate chemistry. The cells are very robust and stable on over charge and at high temperatures. They are very good also for long-term storage at elevated temperatures and have become an important lithium ion chemistry for long-term energy storage and backup applications.
Lithium titanate is a relatively new class of lithium ion. It is characterized by very long cycle life measured in thousands of cycles along with very high discharge and charge rate capability. Lithium titanate is also very safe and comparable to iron phosphate in this regard. The energy density is lower than other lithium ion chemistries and the voltage is 2.4V nominal. There are no standard cell sizes available for Li-titanate currently but the cells generally come in prismatic (some refer to them as “large format”) form factors with 4Ah-20Ah typically.