Machine
learning is a powerful tool to predict the properties of
materials for a variety of applications. However, generating data
sets of carefully characterized materials can be time-consuming and
costly, particularly when numerous candidate materials are later found
to be irrelevant. The problem could be alleviated if machine learning
can be used with minimal information to provide guidance at an early
stage before significant investment has been made. Since structural
characterization is one of the most expensive parts of the process,
this study explores structure-free encoding of materials using Mendeleev
encoding, a method that does not require information such as lattice
constants, lattice positions, or bonding networks. We evaluate Mendeleev
encoding using three data sets of continuous, complex material compounds
used for battery applications, with four different unsupervised learning
methods, inclusive of six algorithms and four evaluation metrics and
in addition visualizations of the results. Our results show that Mendeleev
encoding is more accurate, stable, and reliable than alternative structure-free
encoding, allowing both principle component analysis and archetypal
analysis to capture more of the variance during dimensionality reduction
and consistently provide superior clustering results. Mendeleev encoding
is a simple and scientifically intuitive way of representing material
data that is both human and machine-readable and is applicable to
any machine-learning task training with tabular data.